# IAUS379: Dynamical Masses of Local Group Galaxies

Europe/Berlin
Haus H, Telegrafenberg

#### Haus H, Telegrafenberg

Potsdam, Germany
, ,
Description

Amongst the most stringent limitations in astronomy and cosmology are the very large uncertainties on galaxy masses and sizes. Yet, this topic is undergoing a revolution due to increasing knowledge of 6D space-velocity data for galaxies in the Local Group. This is driven by Gaia and numerous accompanying surveys that provide an accurate Milky Way rotation curve, detailed orbits of the nearby dwarf galaxies, and first estimates of the motions of dwarf irregulars and in the M31 system. This IAU Symposium has a very focused goal: Determining the mass of galaxies from dwarfs to giant spirals. However, it addresses a very broad range of astronomical fields (from studies of variable stars for estimating distances to dynamical modeling), of wave-lengths to be investigated (from radio and optical for estimating the HI gas and stellar content and their motions, to X-rays for estimating the warm and hot ionized gas), and has major implications for a fundamental question (from the distribution and nature of dark matter to alternative models). Measuring galaxy masses at different scales in the Local Group impacts a broad range of astronomy, from stars, star clusters, to the Milky Way, distant galaxies, and Cosmology.

### Key topics:

• Astrometry and 3D velocities: Gaia and HST
• Distances, Chemistry & Star formation in the Local Group: large surveys of resolved stars
• The Galaxy mass and size: rotation curve, halo stars, globular clusters
• Galactic halo dwarf spheroidal and ultra-faint dwarfs
• Magellanic System: the Clouds, the Stream and the Leading Arm
• Dwarf irregular galaxies: dynamics, ram pressure effects
• Andromeda System: rotation curve, halo dwarfs
• Galaxy formation: histories of the Milky Way, of Andromeda, compared to field galaxies
• Near field cosmology: galaxy dynamics, dark matter content, cosmological models

### Meeting information:

• Dates: 20.-24. March 2023
• Location: Potsdam Telegrafenberg, Germany
• Mode: primarily in-person, plus a limited number of online attendance slots will be made available
• Program: a mix of talks, discussions, and poster sessions.
• Workshop fee: 300 Euro (in-person) or 100 Euro (online).
• Registration open until December 31, 2022. Registration deadline extended to January 15 (midnight, CET). After January 15, registrations will only be accepted for virtual participation.
• Fee payment is not due at time of registration but before end of registration period.
• Abstract deadline: October 31, 2022 extended to Friday, November 4, 16:00 CET.

### Invited review speakers

• Giuseppina Battaglia
• Ana Bonaca
• Elena d’Onghia
• Denija Crnojevic
• Federico Lelli
• Alan McConnachie

The SOC plans to select additional invited topical/highlight talks from the submitted abstracts.

### Scientific Organizing Committee

 Francois Hammer (chair) Observatoire de Paris, France Marcel Pawlowski (chair) AIP, Germany Beatriz Barbuy IAG, Brazil Maria-Rosa Cioni AIP, Germany Cuihua Du UCAS, China Anna-Christina Eilers MIT, United States Eva Grebel Heidelberg University, Germany Roger Ianjamasimanana Instituto de Astrofísica de Andalucía (IAA), Spain Stacy McGaugh CWRU, United States Angeles Perez Villegas Instituto de Astronomia-UNAM, Mexico Justin Read University of Surrey, United Kingdom Marina Rejkuba ESO, Germany

### Local Organizing Committee

 Marcel Pawlowski (chair) AIP, Germany Maria-Rosa Cioni (chair) AIP, Germany Salvatore Taibi AIP, Germany Pengfei Li AIP, Germany Kosuke Jamie Kanehisa AIP, Germany Elena Sacchi AIP, Germany Nikolay Kacharov AIP, Germany Mariana Pouseiro Julio AIP, Germany Anna Queiroz AIP, Germany Stefano O. Souza AIP, Germany Yongjun Jiao Observatoire de Paris, France

### Editors of Proceedings

 Piercarlo Bonifacio Observatoire de Paris, France Maria-Rosa Cioni AIP, Germany Francois Hammer Observatoire de Paris, France Marcel S. Pawlowski AIP, Germany Salvatore Taibi AIP, Germany

Registration
IAUS379: Dynamical Masses of Local Group Galaxies
Contact the LOC
• Monday, 20 March
• 09:00 09:15
Welcome Remarks 15m
Speakers: Francois Hammer, Marcel Pawlowski (AIP)
• 09:15 10:45
SESSION 1: From ground and space: motions, distances, chemistry and star formation measurements
Convener: Michelle Collins
• 09:15
The internal kinematics and dynamical masses of Local Group dwarf galaxies 40m

When interpreted within the framework of Newtonian dynamics, the internal kinematic properties of Local Group dwarf galaxies indicate that most of these systems are completely dominated by their dark matter halos. These dwarf galaxies are therefore among the best test-benches for dark matter theories. In this review talk, I will first provide an overview of our current understanding of the internal kinematic properties of Local Group dwarf galaxies, and then discuss the current status of determinations of their dark matter content and distribution, highlighting similarities and differences in the findings from the various methodologies used in the literature. I will also touch upon the 3D motions of Milky Way satellite galaxies
and what they imply for their orbital paths around their host, discussing possible sources of uncertainty at different
levels (e.g. systematics in the proper motion determinations, knowlegde of the gravitational potential of the host).

Speaker: Giuseppina Battaglia (Instituto de Astrofisica de Canarias)
• 09:55
The Power of Milky Way's Stellar Streams with Full 6D+ Phase Space Data Set from S5 and DESI 25m

Stellar streams are one of the most powerful tracers to determine the mass of the Milky Way and other nearby galaxies. Full 6D phase space data is necessary for us to get there. In this talk, I will discuss two ongoing spectroscopic programs to study the stellar streams in our Milky Way and highlight a few latest scientific results from these two programs. The Southern Stellar Stream Spectroscopic Survey (S5), started in 2018, is the first systematic program pursuing a complete census of known streams in the Southern Hemisphere using the fiber-fed AAOmega spectrograph on the Anglo-Australian Telescope. The science results from S5 include a homogeneous study of the kinematic and chemical properties of dozen streams in our Milky Way, the finding of a stream at ~30 kpc possibly perturbed by the dark matter subhalo, the constraints on the mass of the Milky Way and the Large Magellanic Cloud with stellar streams, and the discovery of the fastest hyper velocity stars ejected from Galactic center that can be used to study the shape of the Milky Way halo. The Milky Way Survey of the Dark Energy Spectroscopic Instrument (DESI), on the other hand, is a recently started 5-yr spectroscopic program in the Northern Hemisphere. DESI deploys 5000 fibers over a 3.2 deg diameter field of view at the prime focus of the Mayall 4-meter telescope at the Kitt Peak National Observatory, with a spectral resolution of R~2500-5000 and a wavelength coverage of 3600-9824 Angstrom. With just the first year of data collected in 2021-2022, 3.6 million unique stars have been observed, including many stars in the streams of the northern sky (e.g. GD-1) and showing some interesting features. We are entering an extremely data-rich era in the next decade, with full 6D+chemistry information on dozens of stellar streams, to shape our understanding on the chemo-dynamical evolution of the Milky Way, as well as the nature of the dark matter.

Speaker: Ting Li
• 10:20
Detection of density variations and off-track features in stellar streams 15m

Like any other galaxy in the universe, evidence shows that the assembly of the Milky Way is irrefutably hierarchical. The Galactic halo in particular has a nonlinear structure with a vast number of cold stellar streams with complex morphologies that prove to be a powerful test for the nature and distribution of dark matter in the halo.

In this work, we select halo main sequence stars using Gaia DR3 proper motion and photometry information, the combination of which renders the reduced proper motion parameter. This parameter allows us to pick out high tangential velocity stars in halo orbits independent of the line-of-sight information. Our final catalogue consists of about 47 million halo main-sequence stars for which we can then determine precise photometric distances with typical uncertainties down to 7%. Our sample reaches out until ~20 kpc thereby probing much further out than would be possible using reliable Gaia parallaxes.

Binned velocity moments on the star map in the latitude, longitude and pseudo-azimuth directions pop up several known tidal streams in the local halo - particularly retrograde structures, due to the kinematic selection. The use of main-sequence stars, rather than brighter giants, allows us to trace low surface brightness counterparts, pushing the substructure searches to Gaia’s magnitude limits.

Here I will present the substructures found and characterise them in more detail, due to the added information in derived distances and existing metallicities as well as the higher sensitivity in low surface brightness features. For these streams, we resolve the gaps, wiggles, and density breaks reported in the literature more clearly. The faint signs of disequilibrium in the form of kinks and density variations in these thin streams will paint a more detailed picture of the existence and properties of the dark matter sub-haloes that perturb them and in turn, the mass distribution of our Galaxy.

Speaker: Akshara Viswanathan (Kapteyn Astronomical Institute, University of Groningen)
• 10:35
On the Fast (Stream) Track: Rapid construction of stellar stream paths 2m

Stellar streams are sensitive probes of the Galactic potential. The likelihood of a model given stream data can only be assessed using simulations. However, comparison to simulation is challenging in a noisy 6D phase space in which even the stream paths are hard to quantify. Here we present a novel application of Self-Organizing Maps and first-order Kalman Filters to reconstruct the stream path, propagating measurement errors and data sparsity into the stream path uncertainty. The technique is Galactic- model independent, non-parametric, and works on phase-wrapped streams. We can uniformly analyze and compare data with simulation.

Speaker: Nathaniel Starkman (University of Toronto)
• 10:37
Characterizing the distant spur feature of the Sagittarius stream 2m

Providing a detailed picture of the complexity of the Sagittarius stream is an important aspect of investigating the outer Galactic halo and constraining the Milky Way potential. Several attempts have been made to model the complex structure of the Sagittarius stream. However, no model has yet been able to match all the intricate features observed for the stream, including for instance a bifurcation and several wraps. Most recently, it was discovered that the stream also has a very distant spur feature at least out to ~120 kpc (Sesar et al., 2017).

The aim of this work is to characterize this Sagittarius stream spur using blue horizontal branch stars, one of the few standard candle tracers reaching these outer realms of the Milky Way. Blue horizontal branch candidates were selected using a unique combination of narrow- and broad-band photometry as well as Gaia astrometry. Follow-up optical spectra are obtained using ESO/FORS2. The sample allows us to trace the structure to even larger distances. The observed radial velocities of these stars discriminate different model predictions for the origin of this mysterious spur feature.

Speaker: Manuel Bayer (Kapteyn Astronomical Institute, University of Groningen, the Netherlands)
• 10:39
Bayesian action-based dynamical model for M31 2m

The position, velocity, and chemical composition of each star provide clues to the evolutionary history of galaxies. While the Gaia mission has offered invaluable information about the Milky Way, to understand the growth of disk galaxies in the Universe we must look beyond our galaxy.

The Andromeda Galaxy (M31) is ideal for this task thanks to its proximity (making it possible for individual stars to be resolved) and its inclination angle, providing a gateway to external galaxies study. Furthermore, our position outside of this galaxy is ideal for an unbiased view of its dark matter halo.

I am presenting a novel Bayesian action-based dynamical tool that exploits stars harvested in M31. This pipeline aims to recover parameters of the galaxy describing its distribution function and dark matter density profile. This will be used to understand the galaxy’s accretion history and accumulation of dark matter.

As a first test, the pipeline has been applied to the Auriga simulations of M31-like galaxies. The new Bayesian action-based model recovers well the parameters of the potential and of the distribution function of the halos even when fitted with non-phase mixed, accreted stellar data. Furthermore, this allowed testing the equilibrium assumptions of galaxies and the generality with which double-power law distribution functions can be used to fit stellar halo components of galaxies.

Speaker: Paula Gherghinescu (University of Surrey)
• 10:41
The survey of planetary nebulae in Andromeda (M31). Chemical enrichment of the thin and thicker discs of Andromeda: Oxygen to argon abundance ratios for planetary nebulae and HII regions 2m

We use oxygen and argon abundances for planetary nebulae (PNe) with low internal extinction (progenitor ages of (>4.5 Gyr) and high extinction (progenitor ages <2.5 Gyr), as well as those of the H II regions, to constrain the chemical enrichment and star formation efficiency in the thin and thicker discs of M31. The argon element is produced in larger fraction by Type Ia supernovae (SNe) than oxygen. We find that the mean log(O/Ar) values of PNe as a function of their argon abundances, 12 + log(Ar/H), trace the inter-stellar matter (ISM) conditions at the time of birth of the M 31 disc PN progenitors. Thus the chemical enrichment and star formation efficiency information encoded in the [alpha/Fe] vs. [Fe/H] distribution of stars is also imprinted in the oxygen-to-argon abundance ratio log(O/Ar) vs. argon abundance for the nebular emissions of the different stellar evolution phases. We propose to use the log(O/Ar) vs. (12 + log(Ar/H)) distribution of PNe with different ages to constrain the star-formation histories of the parent stellar populations in the thin and thicker M31 discs. For the inner M31 disc (R_{GC} < 14 kpc), the chemical evolution model that reproduces the mean log(O/Ar) values as function of argon abundance for the high- and low-extinction PNe requires a second infall of metal poorer gas during a gas-rich (wet) satellite merger. In M31, the thin disc is younger and less radially extended, formed stars at a higher star formation efficiency, and had a faster chemical enrichment timescale than the more extended, thicker disc. Both the thin and thicker disc in M31 reach similar high argon abundances ( 12 + log(Ar/H) ) ~ 6.7. The chemical and structural properties of the thin/thicker discs in M31 are thus remarkably different from those determined for the Milky Way thin and thick discs.

Speaker: Magda Arnaboldi (ESO)
• 10:43
(Machine)-learning the galactic potential 2m

Recent observational efforts such as Gaia are leading us toward a new era of data abundance which offers us an incredible opportunity for discovering new physics. Thanks to recent advances in the field of machine learning, it is possible to extract valuable information from the colossal amount of data now available.
In particular, auto-differentiation allows us to get a better grasp of galactic dynamics. It might even enable us to capture a precise and agnostic map of the gravitational potential of the Milky-Way and the underlying dark matter distribution from a mere snapshot of stellar positions and velocities.
However, machine learning in the context of physics is both plagued and blessed by one of its most potent components: neural networks, which are extremely powerful and flexible for modelling physical systems but largely consist in non-interpretable black boxes. Thus, a complementary approach based on symbolic regression is currently being built in the goal of recovering the analytical expression describing a potential. We will present a preliminary study of these new approaches.

Speaker: Wassim Tenachi (Observatoire de Strasbourg)
• 10:45 11:30
Poster Session & Coffee Break 45m
• 11:30 13:00
SESSION 1: From ground and space: motions, distances, chemistry and star formation measurements
Convener: Michelle Collins
• 11:30
Gaia EDR3 proper motions of Milky Way dwarfs 15m

Based on Gaia Early Data Release 3 (EDR3), we estimate the proper motions for 46 dwarf galaxies of the Milky Way. The uncertainties in proper motions, determined by combining both statistical and systematic errors, are smaller by a factor 2.5, when compared with Gaia Data Release 2. We have derived orbits in four Milky Way potential models that are consistent with the MW rotation curve. Although the type of orbit (ellipse or hyperbola) are very dependent on the potential model, the pericenter values are firmly determined, largely independent of the adopted MW mass model. By analyzing the orbital phases, we found that the dwarf galaxies are highly concentrated close to their pericenter, rather than to their apocenter as expected from Kepler's law. This may challenge the fact that most dwarf galaxies are Milky Way satellites, or alternatively indicates an unexpected large number of undiscovered dwarf galaxies lying very close to their apocenters. Between half and two thirds of the satellites have orbital poles that indicate them to orbit along the Vast Polar Structure (VPOS), with the vast majority of these co-orbiting in a common direction also shared by the Magellanic Clouds, which is indicative of a real structure of dwarf galaxies.

Speaker: Hefan Li (National Astronomical Observatories)
• 11:45
Mapping the deforming dark matter haloes of the Milky Way and the LMC with stellar streams 15m

The ongoing merger of the Milky Way and the Large Magellanic Cloud (LMC) is deforming the dark matter haloes of both galaxies, effectively making these galaxies a local dark matter collider. Within this collider, stellar streams act as useful detectors as they are very sensitive to the gravitational potential, and span large parts of the Milky Way halo.
The Orphan-Chenab (OC) stream is particularly insightful as it spans the inner and outer Milky Way, and it passes close to the LMC (within 10kpc). In this talk, I will present the first models of the OC stream in the time-dependent halos of the Milky Way and the LMC that are described by basis function expansions of N-body simulations of the Milky Way-LMC passage. I will show how these deformations have an observable signature on the OC stream. In particular, we find that the Milky Way’s dipole has the most significant effect.
We do not currently have the tools to fit these deformations and ignoring them may result in biases. To test this, we fit mock streams evolved in these deforming potentials with current state-of-the-art stream models. Even though the MW is spherical in these mocks, we infer extremely large flattenings of the DM halos with q=0.6 and q=1.5. This shows that current measurements of the MW DM halo shape are likely biased and motivates the need for computationally efficient tools to describe these deformations. This work is an important first step in measuring the Milky Way’s mass profile over time.

Speaker: Sophia Lilleengen (University of Surrey)
• 12:00
Equilibrium dynamical models for the Large Magellanic Cloud 15m

The Large Magellanic Cloud (LMC) has a complex dynamics driven by both internal and external processes. The external forces are due to tidal interactions with the Small Magellanic Cloud (SMC) and the Milky Way, while internally its dynamics mainly depends on the stellar, gas, and dark matter mass distributions. Despite the overall complexity of the system, very often simple physical models can give us important insights about the main driving factors. Here we focus on the internal forces and attempt to model the proper motions (PM) of ~1 000 000 stars in the LMC as measured by Gaia with an axisymmetric dynamical model, based on the Jeans equations. We test both cored and cusped spherical Navarro-Frenk-White (NFW) dark matter halos to fit the LMC gravitational potential. We find that this simple model is very successful at selecting a clean sample of genuine LMC member stars and correctly predicts the geometry and orientation of the LMC with respect to the observer without additional prior information. Our Jeans dynamical models describes well the mean velocity and velocity dispersion of the LMC stellar disc, however it fails to describe the motions of the LMC bar, which is a non-axisymmetric feature dominating the central region. We plan a triaxial Schwarzschild approach as a next step for the dynamical modelling of the LMC.

Speaker: Nikolay Kacharov (AIP)
• 12:15
Proper Motions, Orbits, and Tidal Influences of Milky Way Dwarf Spheroidal Galaxies 15m

We combine Gaia EDR3 astrometry with accurate photometry and utilize a probabilistic mixture model to measure the systemic proper motion of 52 dwarf spheroidal (dSph) satellite galaxies of the Milky Way (MW). For the 46 dSphs with literature line-of-sight velocities we compute orbits in both a MW and a combined MW + Large Magellanic Cloud (LMC) potential and identify Car II, Car III, Hor I, Hyi I, Phx II, and Ret II as likely LMC satellites. 40% of our dSph sample has a >25% change in pericenter and/or apocenter with the MW + LMC potential. For these orbits, we Monte Carlo sample over the observational uncertainties for each dSph and the uncertainties in the MW and LMC potentials. We predict that Ant II, Boo III, Cra II, Gru II, and Tuc III should be be tidally disrupting by comparing each dSph's average density relative to the MW density at its pericenter. dSphs with large ellipticity (CVn I, Her, Tuc V, UMa I, UMa II, UMi, Wil 1) show a preference for their orbital direction to align with their major axis even for dSphs with large pericenters. We compare the dSph radial orbital phase to subhalos in MW-like N-body simulations and infer that there is not an excess of satellites near their pericenter. With projections of future Gaia data releases, we find dSph orbital precision will be limited by uncertainties in the distance and/or MW potential rather than proper motion precision. Finally, we provide our membership catalogs to enable community follow-up.

Speaker: Andrew Pace (Carnegie Mellon University)
• 12:30
The masses of Local Group galaxies and the baryonic Tully-Fisher relation 15m

The baryonic Tully-Fisher relation is a correlation between the quantity of stars and gas in a galaxy and its flat rotation speed. The relation found in external galaxies appears to hold for rotationally supported Local Group galaxies. It may also hold for some if not all of the pressure supported dwarfs. The flat rotation speed depends on dynamical mass, providing a constraint on total mass that is independent of other considerations.

Speaker: Stacy McGaugh (Case Western Reserve University)
• 12:45
Internal proper motions of dwarf spheroidal galaxies: Constraining the density and properties of dark matter 2m

Determination of the mass density profiles of dwarf galaxies (and specifically whether there is a central core or cusp) provides a critical test of both the properties of dark matter (DM) and the physics of cosmological structure formation. The nearby classical dwarf spheroidal galaxies (dSphs) of the Milky Way yield some of the best dynamical constraints. While large line-of-sight velocity datasets exist (some thousand stars per galaxy), interpretation is hindered by the well-known mass vs. velocity-anisotropy degeneracy of stellar dynamics. This can be resolved with proper motion (PM) measurements that yield 3-D velocity information, which is beyond the reach of Gaia, given the small velocity dispersions of dSphs and the absence of bright stars. To attain the necessary precision and a proper handling of systematics for this kind of study, one then needs not only longer baselines, but the combination of many fields from different state-of-the-art telescopes. We thus obtain separate PMs from the nearby Draco and Sculptor dSphs from 3 epochs of HST data for 5 fields, from 2 epochs of new JWST observations of these same fields as well as two additional fields, all of which are further compared to Gaia DR3 positions. From this long-term program we will be able to provide a direct determination of their velocity anisotropy profiles, and combined with dynamical models, tightly constrain the slopes of their DM density profiles. No comparable measurements exist to date, and the precision attainted will be fundamental to lay more robust constraints on both the nature of DM, and the physical mechanisms that shape DM density profiles in galaxies. We will discuss the progress of our program, and some initial results from the observational epochs and data obtained to date. Additional observations to be obtained through 2025 will yield the final accuracies needed for our goals.

Speaker: Eduardo Vitral (Space Telescope Science Institute)
• 12:47
Constraints on dark matter distribution in dwarf spheroidal galaxies based on the 4th-order Jeans analysis 2m

The mass-anisotropy degeneracy is still one of the main issues in estimating the dark matter distribution in dwarf spheroidal galaxies, especially for the commonly used second-order Jeans analysis. We study the extension of spherical Jeans modeling by incorporating the fourth-order velocity moments under the assumption of dynamical equilibrium and a constant velocity anisotropy. The inclusion of fourth-order velocity moments allows stars’ l.o.s velocity distribution, which is sensitive to the value of the velocity anisotropy parameter, to be flexible, covering thin-tailed to heavy-tailed distributions that is inaccessible if only second-order moments are used. We test our stellar dynamical modeling using mock data that resembles Draco dSph with either central NFW cusp or Burkert core and isotropic velocity anisotropy. Using 500 sample stars, our simulations show that incorporating fourth-order velocity moments improves the results compared to when only second-order moments are used. Typically, the velocity anisotropy is constrained two times better, while it is ~50% improvement for the constraint of the inner dark matter density slope with both parameters being recovered within 1σ uncertainties.

Speaker: M. Dafa Wardana (Astronomical Institute, Tohoku University)
• 12:49
Discrete Schwarzschild models of dwarf spheroidal galaxies: New tests of the dark matter distribution. 2m

An accurate and unambiguous determination of the inner distribution of dark matter in dwarf spheroidal (dSph) galaxies has proven to be a challenge for a few decades now. Some of the complications include the degeneracies inherent to the dynamics of stellar systems, but also the limits of the available data (e.g., just one component of the 3D space velocities of the tracers), and the need for simplifying assumptions of the analysis methods typically employed. In this contribution, we will present initial results from applying our fully discrete axisymmetric Schwarzschild code to this problem. We first study mock datasets for dSph galaxies from the Gaia Challenge, which help us identify the pros and cons on the tool we use, and understand the limits imposed by the degeneracies inherent to the problem. We explore datasets of various sizes, and all combination of available velocity components, i.e., only line-of-sight velocities, only proper motions, and both. Our methodology also avoids restrictive assumptions about the degree of orbital (an)isotropy. We then apply the tool to line-of-sight velocity data for the Sculptor and Fornax dwarfs, and compare our preliminary results to published work.

Speaker: Erik Aquino Ortíz (Instituto de Astrofísica, Pontificia Universidad Católica de Chile)
• 12:51
The Halo Outskirts With Variable Stars (HOWVAST) survey: detecting Milky Way mass tracers beyond 100 kpc 2m

According to our currently favored cosmological framework, the Lambda- Cold Dark Matter model, galaxies like the Milky Way were built through the accretion and merger of smaller systems. In this scenario, the extended halo of the Milky Way must retain information about this process. Key to unveiling this information is our ability to trace the outermost regions of the halo by detecting and studying the properties of bound objects. One of the most relevant family of such objects is the low-mass, old-population, RR Lyrae (RRL) pulsational variables, ubiquitous in the halo and for which precise distances can be obtained. RRLs have proven to be essential in related but different areas: in this particular, we seek to detect distant (>100 kpc) RRLs that can help improve estimations of the Milky Way mass, especially at large distances. In this context, we report our search for faint RR Lyrae stars using Dark Energy Camera (DECam) data over ~400 sq. deg., as part of the Halo Outskirts With Variable Stars (HOWVAST) survey, where we detect more than ∼500 RR Lyrae candidates ranging in heliocentric distances from 7 to 270 kpc. 27 of these stars are located beyond 100 kpc from the Galactic center, increasing our current sample of distant mass tracers, critical to improve current Milky Way mass determinations which suffer from at least 50% uncertainty. HOWVAST represents our effort to carry out frontier Galactic science done with RRLs, and should only be surpassed once the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) begins scientific operations, which is expected for mid 2024.

Speaker: Ricardo R. Munoz (Universidad de Chile)
• 12:53
Precise distances from Gaia astrometry and large-scale spectroscopic surveys with StarHorse 2m

The ESA's astrometric mission Gaia has added an invaluable wealth of astrometric and photometric data for more than a billion stars in our Galaxy (Gaia Collaboration et al. 2018). The synergy between Gaia's third data release, EDR3, and large scale-spectroscopic surveys give us comprehensive information about individual stars in the Milky Way. To complement these data sets, we deliver new catalogues (10 million stars) of distance, extinction, masses, and additional parameters produced with the Bayesian isochrone-fitting code StarHorse (Queiroz et al. 2018). These results are crucial for the study of Milky Way dynamics and the characterization of disrupted dwarf galaxies; we show applications of our produced data to the study of Sagittarius and Gaia Enceladus. The resulting catalogues are essential to model the Milky Way's chemo-dynamical history and further understand the formation of disk galaxies.

Speaker: Anna Queiroz (AIP)
• 13:00 13:30
Poster Session 30m
• 13:30 14:30
Lunch Break 1h
• 14:30 15:25
SESSION 1: From ground and space: motions, distances, chemistry and star formation measurements
Convener: Tony Sohn
• 14:30
Chemo-dynamical modelling of galaxies with DYNAMITE 15m

The combination of astrometric and chemical information from Galactic stars has revealed great detail about the structure, dynamics and history of our own Galaxy. In external galaxies, it is impossible to map the distribution of individual stars, but high signal-to-noise integral field spectroscopy data at various wavelengths, together with sophisticated dynamical models, give us the opportunity to gather information on the structure, dynamics and history of these systems.
I will present the Schwarzschild code DYNAMITE (https://dynamics.univie.ac.at/dynamite_docs/index.html), which models galactic dynamics by means of a superposition of stellar orbits. DYNAMITE is equipped to deal with very detailed kinematic measurements, allowing us to better exploit high-quality IFU datasets of nearby galaxies. I will provide a detailed overview of the challenges of such a modelling technique and introduce first applications on observations and simulations. I will also show how DYNAMITE makes it possible to identify families of orbits originating from different dynamical structures within a galaxy, which in some cases challenge the current picture of galaxies obtained from pure stellar light decompositions. Finally, I will anticipate our future plans for DYNAMITE, adding additional tracers (globular clusters, extended gas), and dealing with discrete data.

Speaker: Alice Zocchi (University of Vienna)
• 14:45
How 4MOST and MOONS will improve constraints on the dynamical masses of Local Group galaxies 15m

The wide-field spectrographs 4MOST and MOONS will enter operations in 2024 at the ESO Paranal Observatory. These upcoming survey facilities will play an important role in various fields over the next decade. In particular, both will host surveys aimed at observing nearby Local Group galaxies. Here, we provide an overview of all the planned 4MOST and MOONS surveys that focus on Local Group galaxy kinematics. We will describe how their scientific performances complement Gaia and other spectroscopic surveys in the field of nearby galaxy kinematics. In addition, we will outline the outstanding scientific questions regarding the local group galaxy masses and dynamics, in which the MOONS and 4MOST survey science will allow major progress. Finally, we will describe how the community can access observing time and data, to participate in solving fundamental issues in this field.

Speaker: Steffen Mieske
• 15:00
Wide binaries in dwarf galaxies as probes of dark matter 25m

Very wide stellar binaries, with semi-major axes of hundreds of AU and larger, constitute sensitive probes of the underlying gravitational potential in which they live, having provided some of the first experimental constraints on the nature of dark matter in the Milky Way halo.  As such, the detection and characterization of populations of wide binaries in nearby old dwarf galaxies could provide us with a completely new window onto the properties of dark matter on the smallest scales. We have designed a new method, geometric in its essence, for deriving the density and projected semimajor-axis distribution of a population of binary systems that does not resort to standard estimators of the two-point correlation function, and will present results of applying it to our Hubble Space Telescope survey of the Ursa Minor dwarf spheroidal galaxy.  We will discuss the competing effects of the survey's depth, coverage, and angular resolution limits against those of foreground stars and background galaxies, and highlight the potential advantages that JWST and the Roman Space Telescope may offer to this specific problem.

Speaker: Christopher Kervick (Carnegie Mellon University)
• 15:25 16:00
Plenary Discussion 1 35m
Speakers: Alexander Ji (University of Chicago), Roger Ianjamasimanana
• 16:00 16:45
Poster Session & Coffee Break 45m
• 16:45 18:00
SESSION 2 : Sizes, Masses, and Formation histories of the Milky Way and of Andromeda
Convener: Tony Sohn
• 16:45
Scaling up the Milky Way 40m

Our Galaxy, the Milky Way, provides us with a unique opportunity to measure the 3D shape of its dark-matter halo, thus testing the properties of the dark sector and the fundamental cosmological model. Until recently, however, these efforts have been thwarted by the scarcity of observational data, especially in the outer halo, as well as reliance on the assumption of a dynamical equilibrium, resulting in estimates of the total Milky Way mass spanning a factor of a few. I will discuss how the 6D phase-space data from the Gaia mission and ground-based surveys, combined with novel modeling methods, have been used to precisely map not just the Milky Way's halo at the present, but also reconstruct its accretion history.

Speaker: Ana Bonaca (Carnegie Observatories)
• 17:25
DESI and the matter density distribution of the Milky Way from field halo stars 15m

The Dark Energy Spectroscopic Instrument (DESI) is currently one of the most powerful instruments for wide-field multi-object spectroscopy. The synergy of DESI with current (e.g. ESA’s Gaia satellite) and future observing facilities including the Vera Rubin Observatory’s Legacy Survey of Space and Time (LSST), and the Nancy Grace Roman Space Telescope’s High Latitude Survey (HLS) will yield datasets of unprecedented size and coverage that will enable strong constraints on the dark matter distribution in the Milky Way (MW) and Local Group galaxies including M31. By the end of 2024 DESI spectra (350nm-980nm) will be obtained for 7.2 million stars in the main program and for up to another 5 million stars via the backup program (bright/poor sky condition) in the MW alone. As of May 2021 (when Kitt Peak shut down due to the Conteras Fire) DESI had already obtain spectra of 3.6 million unique stars. After a brief introduction to the DESI MW survey, its spectroscopic pipeline and what to expect in the early data release (mid 2023), I will present results from two new modeling codes that have been recently developed to constrain the mass distribution of the MW that utilize the full 6D as well as 5D phase space data. First I will describe a new B-spline based non-parametric spherical Jeans modeling code (NIMBLE, Rehemtulla et al. 2022). Tests of NIMBLE with mock data from the Latte cosmological simulations show that it is possible to constraint the mass of the MW out to ~80kpc with ~15\% accuracy even in the presence of halo substructure and moderate amounts of disequilibrium. I will then show preliminary results of the application of NIMBLE to Survey Validation data from DESI. Finally, I will describe results from an axisymmetric distribution function fitting code (Hattori et al. 2021) and results from its application to Gaia RRLyrae data.

Speaker: Monica Valluri (University of Michigan)
• 17:40
Dynamical properties of ancient stars in the inner Milky Way with PIGS 15m

The oldest, most metal-poor stars in the Milky Way are unique probes of early star formation and the assembly of the Milky Way. The Galactic bulge region has typically been avoided in the search for metal-poor stars, because of the extremely high density of mostly metal-rich stars and the high dust extinction. The bulk of the Galactic bulge is thought to originate from the (early) Galactic disk. However, the oldest pressure-supported component in our Galaxy is also expected to be present in its innermost region. The most metal-poor stars in the bulge region can provide unique insights into the ancient Milky Way.

I will present results from the Pristine Inner Galaxy Survey (PIGS), which used metallicity-sensitive narrow-band CaHK photometry to identify and follow up spectroscopically thousands of metal-poor candidates in the bulge. Using the bulk PIGS radial velocities, we previously showed that the amount of inner Galaxy rotation decreases with decreasing metallicity. I will present recent work on the detailed orbital properties for all stars in PIGS, which strongly depend on their metallicities and broadly show a transition from a disky bulge to a pressure-supported component. It is exciting to see the growing amount of data on metal-poor stars in the inner regions of the Milky Way, thanks to various surveys, allowing us to set important constraints on the formation of the ancient inner Galaxy.

Speaker: Anke Arentsen (Institute of Astronomy, Cambridge)
• Tuesday, 21 March
• 09:00 10:45
SESSION 2 : Sizes, Masses, and Formation histories of the Milky Way and of Andromeda
Convener: Alan McConnachie
• 09:00
The Milky Way Tomography with Subaru Hyper Suprime-Cam. I. Halo substructures 15m

Under the current standard scenario, galaxies such as the Milky Way (MW) are thought to have formed through repeated mergers and accretions of small galaxies due to gravitational interactions. Since the relaxation timescale of these traces in phase space is as long as more than 10 billion years, it has been theoretically suggested that they may still exist in the halo. Advances in large-scale observations have ushered in the era in which such traces can be investigated observationally. The MW, to which we belong, is of particular interest because it is possible to directly observe stellar streams and substructures as they are destroyed by tidal forces as small galaxies merge and accrete into it. Currently while the structure and substructures in the halo within 20-30 kpc from the center of the MW is becoming better understood from the viewpoint of chemodynamics, the structure in the outer halo is still largely unresolved.
In this study we used the entire HSC-SSP data (~1400 square degrees), characterized by wide field of view (~1.8 square degrees) and deep photometry (i < 26 mag), to investigate the outer halo of the MW. In order to effectively detect the substructures in the outer halo, an isochrone-filter was created for the old, metal-poor stellar systems on the color-magnitude diagram. As a result, previously discovered substructures (e.g. the Orphan Stream) were detected, while new candidates were discovered. In this talk, we will discuss the origin of these substructures.

Speaker: Yoshihisa Suzuki (Astronomical Institute, Tohoku University)
• 09:15
The study of the Galactic mass distribution from Gaia DR3 RR Lyrae 15m

The mass of the Milky Way (MW) is important to the formation and evolution of galaxy. After decades of study, the mass of dark matter halo is still open. Most studies have used dynamical tracers in the inner regions of the halo, relying on extrapolations to estimate the mass of the MW. In our study, we determine the Milky Way mass distribution from fitting dynamical models to the gravitational force field and the Galactic rotation curve. Based on RR Lyrae with accurate proper motions and classification in Gaia DR3, we obtain the distance with relative uncertainty less than 5% by using the extinction-free Period-Wesenheit relation. Applying Gaussian Mixture Model to the intrinsic velocity distribution, we present the result of a multi-component kinematic model of RR Lyrae in the inner regions. Considering the early accretion history of the MW so that the stellar halo may not be in equilibrium, we separate the halo population into an isotropic stellar halo and the radially-anisotropic population relevant to a merge event. With a Bayesian method, we fit the potential model parameters, including the density flattening of the dark matter (DM) halo.
In this talk, I shall discuss our best-fitting results about the shape and DM halo mass.

Speaker: Cuihua Du
• 09:30
Milky Way mass with K giants and BHB stars using LAMOST, SDSS/SEGUE, and Gaia: 3D spherical Jeans equation and tracer mass estimator 25m

We measure the enclosed Milky Way mass profile to Galactocentric distances of $\sim70$ and $\sim50$ kpc using the smooth, diffuse stellar halo samples of Bird et al. The samples are LAMOST and SDSS/SEGUE K giants (KG) and SDSS/SEGUE blue horizontal branch (BHB) stars with accurate metallicities. The 3D kinematics are available through LAMOST and SDSS/SEGUE distances and radial velocities and {\it Gaia} DR2 proper motions. Two methods are used to estimate the enclosed mass: 3D spherical Jeans equation and Evans et al. tracer mass estimator (TME). We remove substructure via the Xue et al. method based on integrals of motion. We evaluate the uncertainties on our estimates due to random sampling noise, systematic distance errors, the adopted density profile, and non-virialization and non-spherical effects of the halo. The tracer density profile remains a limiting systematic in our mass estimates, although within these limits we find reasonable agreement across the different samples and the methods applied. Out to $\sim70$ and $\sim50$ kpc, the Jeans method yields total enclosed masses of $4.3\pm0.95$ (random) $\pm0.6$ (systematic) $\times10^{11}$ M$_\odot$ and $4.1\pm1.2$ (random) $\pm0.6$ (systematic) $\times10^{11}$ M$_\odot$ for the KG and BHB stars, respectively. For the KG and BHB samples we find a dark matter virial mass of $M_{200}=0.55^{+0.15}_{-0.11}$ (random) $\pm0.083$ (systematic) $\times10^{12}$ M$_\odot$ and $M_{200}=1.00^{+0.67}_{-0.33}$ (random) $\pm0.15$ (systematic) $\times10^{12}$ M$_\odot$, respectively.

Speaker: Sarah A. Bird
• 09:55
The Milky Way's Rotation Curve Derived from Classical Cepheids 2m

Classical Cepheids are excellent tracers to estimate the rotation velocity of the galaxies because they provide better distance accuracy with less uncertainty. With stringent radial velocity from the recent Gaia DR3 and proper motion, we estimate the rotation velocity of the Milky Way galaxy for 909 Classical Cepheids. We have used a more accurate distance estimated based on the period-luminosity from a mid-infrared survey. Up to 20 kpc, we find the best-fit rotation velocity of 240.15 ± 1.95 km/s (R − R_0) and a concentration parameter of 17.66 ± 0.09. Also, we obtain the virial mass of (5.73 ± 0.04 × 10^11)M_0.

Speaker: Raj Kumar Pradhan (Central Department of Physics, Tribhuvan University)
• 09:57
The relation between stellar and dark matter haloes of Milky Way-mass galaxies 2m

In this talk, I will discuss the predictions of the $\Lambda$CDM model on the make-up of stellar and dark matter haloes of Milky Way-mass galaxies. I will show that the stellar haloes are made up largely of stars from massive mergers, like Gaia-Enceladus/Sausage in the Milky Way, while the same mergers also make up nearly half of the dark matter halo. Past mergers leave imprints in the phase-space structure of the stellar halo. In particular, the "edge" of the stellar halo is defined by the latest major merger and the location of the "edge" is indicative of the total mass and recent assembly history of the galaxy. I will also show that recently accreted massive satellites (analogous to the LMC) contribute nearly a quarter of the total dark matter mass of Milky Way-mass galaxies despite not being fully disrupted yet.

Speaker: Anna Genina
• 09:59
Milky Way Modelling with Stellar Streams 2m

Stellar streams are created when globular clusters or dwarf galaxies tidally disrupt in the gravitational potential of their host galaxy. These streams therefore offer a great probe to this galactic potential. Current observations show a multitude of Milky Way stellar streams to have substructure in the form of 'spurs', 'gaps' and even multiple components. These can originate from interactions of the stream with dark matter subhalos, which could probe the nature of dark matter. I'll talk about the specific case of the Jhelum stream, which shows multiple components after a close interaction with the Sagittarius Dwarf Galaxy, which means we should take the classical satellites into account when modelling stellar streams to infer information about the Milky Way potential.
Another recent observation (Dodd et al 2022a) shows that the Helmi Streams seem to orbit near one or multiple resonances. This results in substructure in angular momentum space. To explain the persistence of this substructure, one seems to need a prolate dark matter halo in the region where the Helmi streams orbit. If there is some time left, I'll talk about this substructure when considered in the context of an alternative theory of gravity called MOND (Modified Newtonian Dynamics). I'll introduce my simple MOND models of the Milky Way and discuss their ability to sustain this Helmi Streams substructure.

Speaker: Orlin Koop (Kapteyn Astronomical Institute)
• 10:01
Are the orbital poles of the Milky Way streams pointing to a past Milky Way-Andromeda interaction? 2m

Understanding the origin of the stellar streams around the Milky Way can be of great relevance to learn about the history of the Milky Way and the formation of its substructures. A previous study on the Milky Way streams (Pawlowski et al. 2012) showed that many of these (7 out of 14) present a similar orientation to that of the disk of satellite galaxies (DoS) and the young globular clusters of the Milky Way. This suggests that the DoS, the young globular clusters and a large fraction of the Milky Way streams have a correlated origin. The authors named this group of subsystems the Vast POlar Structure" (VPOS), and proposed that it could have formed as a result of a past interaction between the Milky Way and the Andromeda galaxy. A more recent study (Riley et al. 2020) analysed 64 Milky Way streams and concluded that there is no evidence of clustering around the VPOS direction in the orbital poles of the Milky Way streams once the newly discovered streams are taken into account. In this work, we revise the distrubution of the orbital poles of the Milky Way streams in light of the latest stream dataset, which includes a total of 97 streams, improved measurments of their positions and dynamics and a more reliable method for obtaining their orbital poles.

Speaker: Elena Asencio (University of Bonn)
• 10:03
Disrupted vs survived dwarf galaxies in cosmological simulations 2m

Since the chemical abundances of stars are the fossil records of the physical conditions in galaxies, they provide the key information for recovering the assembly history of galaxies. In this work, we explore the chemo-chrono-kinematics of accreted and survived dwarf galaxies by analyzing six M31/MW analogues from the HESTIA suite of cosmological hydrodynamics zoom-in simulations of the Local Group. We found that accreted stellar haloes, including individual debris, reveal abundance gradients in the E-Lz space, where the most metal-rich stars have formed in the inner parts of the disrupted systems before the merger and mainly contribute to the central regions of the hosts. Therefore, we suggest that abundance measurements in the inner MW will allow to constrain better the parameters of building blocks of the MW stellar halo. We found that the merger debris are chemically distinct from the survived dwarf galaxies; however, the mergers debris have abundances expected for stars originating from dwarfs that had their star formation activity quenched at early times. Using the data from the APOGEE spectroscopic survey we also explore some other similarities between satellite galaxies and accreted stellar halo of the MW.

Speaker: Sergey Khoperskov (AIP)
• 10:05
Weighing Andromeda: Mass estimates of the M31 Galaxy 40m

Andromeda (M31) is the nearest giant spiral galaxy to the Milky Way and the most massive member of the Local Group. It has long been recognized that M31's mass measurement is essential to understand the formation and evolution of the Local Group. I will review the different observational and modelling techniques that have developed over time to measure the mass of M31. I will discuss the the best constraints today of the M31 mass and the consistency of that obtained from different techniques. I will also discuss possible improvements on the M31 mass measurement from improved techniques driven by data from forthcoming instruments and surveys.

Speaker: Souradeep Bhattacharya (Inter University Center for Astronomy and Astrophysics (IUCAA), Pune, India)
• 10:45 11:30
Poster Session & Coffee Break 45m
• 11:30 13:13
SESSION 2 : Sizes, Masses, and Formation histories of the Milky Way and of Andromeda
Convener: Alan McConnachie
• 11:30
Proper Motions of M31 Satellite Galaxies: M31 Mass, Satellite Orbits, and the Dynamical State of the Satellite Plane 25m

Proper motions (PMs) from HST and Gaia have revolutionized the field of Galactic archaeology in the Milky Way (MW). However, PM studies in and around our neighbor spiral galaxy M31 are still in their early stages with measurements being available for only a few satellites and M31 itself at the moment. Gaia can only detect some of the brightest stars in star forming regions at the distance of M31, so HST and JWST remain the only reliable options for measuring PMs of M31 satellite dwarf spheroidals. Three-dimensional kinematics based on PM measurements of satellites will allow constraining the total mass of M31 and testing the dynamical stability of the Great Plane of Andromeda. Furthermore, orbital histories of individual satellites based on the PM results will provide clues to disentangle the processes that have contributed to the quenching of star formation and help connect some satellite galaxies to M31 substructures. In this contribution, I will present the progress of our HST and JWST programs to measure PMs of M31 satellite galaxies.

Speaker: Tony Sohn (Space Telescope Science Institute)
• 11:55
Constraining the Total Mass of M31 with Precision Astrometry 15m

High-precision astrometric data from the Hubble Space Telescope (HST) and Gaia are revolutionizing our ability to study the Local Group. Currently, 6D phase space measurements (3-dimensional position and velocity) are available for a majority of the Milky Way’s known satellite galaxies and for four (11%) of M31’s satellite galaxies. As satellites trace the dark matter halos of their hosts, often, the dynamical properties of a given satellite are used to constrain the mass of the Milky Way (MW) or M31. However, my recent work has shown that using the 6D phase space information for an ensemble of satellite galaxies simultaneously can significantly reduce the current factor of two uncertainty in the mass range of the MW. In this talk, I will describe how dynamical properties derived from 6D phase space information of four M31 satellites (M33, IC 10, NGC 147, NGC 185) can be used in combination with state-of-the-art cosmological simulations to statistically estimate the mass of M31, reducing current uncertainties to 30-60%. Over the next decade, HST will deliver astrometric data for the remainder of M31’s satellite population. Applying these methods to the full population of satellites out to ~300 kpc will yield the most precise and complete M31 mass estimate to date. This will be a crucial result for interpreting the severity of classical small-scale LCDM challenges (i.e. missing satellites, too-big-to-fail), the assembly history of M31, and the fate of the Local Group.

Speaker: Ekta Patel (UC Berkeley)
• 12:10
Probing the mass of the Andromeda galaxy with DESI spectroscopy 15m

We present DESI observations of the stellar halo of M31 which reveal the kinematics of a recent merger in exquisite detail. Using data from less than four hours of observations by DESI survey we measure radial velocities of more than 7000 sources in M31. These observations show an intricate coherent kinematic structure in position and velocity space in M31 stellar halo. While hints of coherent structures have been previously detected in M31, this is the first time they have been seen with such detail and clarity in a galaxy beyond the Milky Way. We find clear kinematic evidence for multiple shell structures across M31. The kinematics of these structures are remarkably similar to the predictions of dynamical models of a single merger event that happened ~2 Gyr ago and enable us to constrain the gravitational potential of the M31. Using a model of just a small part of the merger debris observed by DESI we estimate the total mass of M31 within a projected radius of 125 kpc to be log10(M/Msun) = 11.78+/-0.1. This study opens a new era in our ability to map stellar halos and measure masses of nearby galaxies with high multiplex spectrographs.

Speaker: Sergey Koposov (Institute for Astronomy, University of Edinburgh)
• 12:25
Formation of the Andromeda Giant Southern Stream and the ring structures in the Andromeda Galaxy 15m

Recent observations around the M31 have revealed many traces of past interactions with satellite galaxies. In particular, the Andromeda Giant Southern Stream (AGSS) in the halo and the double ring structure in the disc have been drawing attention. The AGSS is a giant structure extending more than 100 kpc from the center of M31 and is thought to have been formed in a collision with a satellite galaxy about 80 billion years ago (Fardal et al. 2007; Mori & Rich 2008). On the other hand, Block et al. (2006) found the double ring structure in M31 made up of gas and dust and argued that the structure was formed by a head-on collision of a satellite galaxy, M32, around 20 billion years ago. They conclude that the mass of M32 at the time of the collision was about one-tenth of the total mass of M31 $\sim 10^{11} M_\odot$. Moreover, a model has been proposed by Hammer et al. (2018) in which the AGSS and the double ring structure are formed simultaneously by a major merger with the mass of more than $10^{11} M_\odot$ for the first passage at the large pericentric distance.
These situations motivate us to investigate the possible link between the AGSS and the 10kpc ring structures using $N$-body/SPH simulations of a miner merger between the M31 and a satellite galaxy with a mass of $10^{10} M_\odot$. The simulation result successfully matches the observed features of the AGSS and the 10 kpc rings concurrently. The stars are smoothly distributed in the galactic disk, but there are some rings of gas and dust that reproduce the observations. In addition, we demonstrate the spatial metallicity distribution of the merger remnants, assuming the progenitor galaxy's metallicity gradient. The result remarkably captures the observed features in the AGSS exhibiting non-uniform metallicity distribution perpendicular to the AGSS axis (Preston et al. 2021). These results indicate that a minor merger of the massive dwarf galaxy is also capable of simultaneously forming the AGSS and the 10 kpc ring.

Speaker: Masao Mori (University of Tsukuba)
• 12:40
Galaxy masses derived from the timing argument 25m

The rather intuitive concept of 'galaxy mass' is an ill-defined quantity in cosmology. First, because in an expanding, close-to-homogeneous Universe collapsed structures do not show well-defined boundaries, and second because the availability of dynamical tracers becomes very scarce in the outskirts of dark matter haloes. In this talk I will provide an overview of the timing argument, which models the relative motion of massive substructures in an expanding Universe as a restricted 3-body system. I will show that this method returns masses that are systematically higher than the mass enclosed within the nominal virial radius of a galaxy, thus complicating a direct comparison with 'dynamical' masses derived from halo tracers. As an application, I will summarize recent attempts to measure simultaneously the masses of our Galaxy, the Large Magellanic Cloud and M31.

Speaker: Jorge Peñarrubia (University of Edinburgh)
• 13:05
Exploring the dynamical decoupling of the components of the Andromeda galaxy 2m

The study of the dynamical mass of the Local Group requires a detailed knowledge of the velocity of its elements, in particular of the Milky Way (MW) and the Andromeda galaxy (M31). Nevertheless, a discrepancy between the proper motion of the disk of M31 and the global motion of its satellites has been identified. Moreover, recent results showing the influence of the Magellanic complex on the displacement of the MW disk suggest a possible decoupling between the different components of spiral galaxies, notably the dark and baryonic parts. To explore this possibility, we study the position and kinematic deviations that may arise between the disk of a MW (or M31)-like galaxy and its halo, from constrained high resolution cosmological simulations of the Local Group in realistic environment, namely HESTIA simulations. We focus on the 3-dimensional analysis of the centers of mass (COM). We present two parts. We first consider individual particles to track down the very nature and amplitude of the physical deviations of the COM with respect to the distance from the disk center. Dark matter is dominating the behavior of the COM of all particles at all distances. But the total COM is also very close to the COM of stars. In the absence of a significant merger, the velocity offsets are marginal (10 km/s) but the positional shifts can be important compared to the disk characteristics (> 10 kpc). In the event of a major merger, discrepancies are found to be of the same order as the recent finding for the MW under the Magellanic Clouds influence. In a second part, we put the accent on the study of various populations of subhaloes and satellites. We show that while satellites properly represent the entire subhalo population, there exists strong mismatch in phase space between their COM and the host disk. Moreover, the results are highly inhomogeneous between the simulations, and thus between the accretion histories. It is highlighted that these shifts are mainly due to the three or four most massive objects. We will finally compare these results in the light of our observational knowledge.

Speaker: Jean-Baptiste Salomon (Hebrew University)
• 13:07
Detection limits and estimation of Andromeda's mass 2m

The large uncertainties on the measurement of the mass of galaxies is an important issue in modern astrophysics. For both the MW and M31, thanks to the gas rotation curve, the mass of the inner part of the haloes is well constrained. For the outer part of the haloes, we must turn to satellites galaxies as tracers. But, to use dwarf galaxies as such, it is absolutely crucial to determine the dwarf galaxy detection limits so they can be accurately modeled into the dwarf galaxy system models. I will present the first such effort to characterize fully the dwarf galaxy system of the Andromeda galaxy, based on the PAndAS photometric mapping. As expected, the detection limits are a strong function of the size, luminosity and the location of a dwarf galaxy in the survey. I will then present the impact of such completeness on the determination of M31 mass.

Speaker: Amandine Doliva-Dolinsky (Observatoire Astronomique de Strasbourg)
• 13:09
Revisiting mass estimates of the Milky Way, of M31, and of some spiral galaxies 2m

We estimate that the mass of the Milky Way (MW) is in the range of 2 to 15 $\times 10^{11} M_\odot$. It results from an analysis of the rotation curve (RC) from Gaia DR2 and using different profiles for baryon and dark matter (DM, including NFW & Einasto profile). The lower limit 2 $\times 10^{11} M_\odot$ corresponds to the Keplerian slope of RC at large radii. There were no major mergers in the MW since 9 $\sim$10 Gyr ago and then the dynamical mass is well established from its RC.

However, most distant galaxies have been found in the process of merging, which perturbs the motion of stars and gas. Thus it is essential to determine if the galaxy outskirts could be at equilibrium after the merger epoch. Using a library of the simulations of galaxy major-merger (including M31), we will study the criteria for determining the relaxation of galactic disks. We will apply these criteria to the nearby spiral galaxies and determine the relaxation time scale and radius. Then we can measure robust dynamical masses within a given radius.

Speaker: Yongjun JIAO (Paris Observatory)
• 13:11
Large-scale Hydrodynamical Shocks as the Smoking-gun Evidence for a Bar in M31 2m

The formation and evolutionary history of M31 are closely related to its dynamical structures, which remain unclear due to its high inclination. Gas kinematics could provide crucial evidence for the existence of a rotating bar in M31. Using the position–velocity diagram of [O III] and H I, we are able to identify clear sharp velocity jump (shock) features with a typical amplitude over 100 km/s in the central region of M31 (4.6 kpc × 2.3 kpc, or 20' x 10'). We also simulate gas morphology and kinematics in barred M31 potentials and find that the bar- induced shocks can produce velocity jumps similar to those in [O III]. The identified shock features in both [O III] and H I are broadly consistent, and they are found mainly on the leading sides of the bar/bulge, following a hallmark pattern expected from the bar-driven gas inflow. Shock features on the far side of the disk are clearer than those on the near side, possibly due to limited data coverage on the near side, as well as to obscuration by the warped gas and dust layers. Further hydrodynamical simulations with more sophisticated physics are desired to fully understand the observed gas features and to better constrain the parameters of the bar in M31.

Speaker: Zixuan Feng (Shanghai Astronomical Observatory)
• 13:13 13:30
Poster Session 17m
• 13:30 14:30
Lunch Break 1h
• 14:30 15:10
SESSION 2 : Sizes, Masses, and Formation histories of the Milky Way and of Andromeda
Convener: Philipp Richter
• 14:30
The Triangulum Extended Survey: Insights into the Dynamical History of M33 and M31 15m

Triangulum (M33), a satellite of the Andromeda (M31) galaxy, is the only dwarf Spiral in the Local Group. With a mass ten times lower than M31’s and a star formation rate 10 times higher, M33 is the best local analog for high z galaxies. The Triangulum Extended Survey (TREX) is a large resolved stellar spectroscopic survey of M33 and its extended structures. With contiguous spectroscopic fields covering M33's inner disk, out to M33's disk break, and beyond, we are investigating the evidence for both internal and external heating mechanisms affecting M33. Using a sample of over 4500 M33 stars with line of sight velocity measurements, spanning from young, massive main sequence stars to old red giant branch stars, we established that a significant high-velocity-dispersion component is present in M33's RGB population from near M33's center to at least the radius where M33's H I disk begins to warp at 30' (~7.5 kpc) in the plane of the disk. This is the first detection and spatial characterization of a kinematically hot stellar component throughout M33's inner regions. However, beyond the break in M33's disk, we find the stellar population is dominated by stars with disk-like kinematics, with only marginal evidence for a kinematically hot halo component, casting doubt onto whether this component is likely to have been formed from accretion of smaller systems. We have also measured the velocity dispersion and asymmetric drift of stars in the disk as a function of stellar age, finding neither increases with stellar age and the youngest disk stars are dynamically hotter than predicted by simulated M33 analogs in Illustris. This indicates an additional, currently unknown source of dynamical heating of the young stars in the disk of M33. I will discuss these TREX results, as well as future prospects, in the context of our understanding of the orbital history of M33, in particular, the question of whether M33 is on first infall or has interacted substantially with M31 in the past.

Speaker: Karoline Gilbert (Space Telescope Science Institute)
• 14:45
The Mass of the Local Group 25m

We review estimates of the total mass of the Local Group. High-accuracy proper motions (PMs) of M31 and other Local Group (LG) satellites have now been provided by the Gaia satellite. We revisit the timing argument to compute the total mass of the LG from the orbit of the Milky Way and M31. We discuss a number of systematic effects. The first is caused by the presence of the Large Magellanic Cloud (LMC). The interaction of the LMC with the Milky Way induces a motion toward the LMC. This contribution to the measured velocity of approach of the Milky Way and M31 must be removed. The second is the cosmological constant whose effects must be incorporated on these length scales. The third is to allow for cosmic bias and scatter, pre-conditioned by the accretion history of the LG. Without taking these into account, the timing argument significantly overestimates the true mass. Adjusting for all these effects, we give the estimated mass of the LG for two treatments of M31's tangential velocity. The first is $M =3.4^{+1.4}_{−1.1} \times 10^{12}M_⊙$ (68% CL) when using the M31 tangential velocity $82^{+38}_{−35}$ km/s. Lower tangential velocity models with $59^{+42}_{−38}$ km/s (derived from the same PM data with a flat prior on the tangential velocity) lead to an estimated mass of $M=3.1^{+1.3}_{−1.0}×10^{12}M_⊙$ (68% CL). By making an inventory of the total mass associated with the four most substantial LG members (the Milky Way, M31, M33, and the LMC), we estimate the known mass to be in the range $3.7^{+0.5}_{−0.5} \times 10^{12}M_⊙$

Speaker: Wyn Evans (Cambridge University)
• 15:10 16:10
Plenary Discussion 2 1h
• 16:10 16:55
Poster Session & Coffee Break 45m
• 16:55 17:55
SESSION 3 : Dwarf galaxies: dwarf spheroidal, ultra-faint dwarfs, dwarf irregulars
Convener: Philipp Richter
• 16:55
On the many observational challenges to estimating the masses of dwarf galaxies 40m

I will briefly summarise the key methods for the determination of the dynamical mass and mass profiles of dwarf galaxies. I will then discuss in detail the observational challenges associated with making the necessary measurements and obtaining the necessary data, with a focus on the systematic, technical, and physical limitations of the measurement. I will review the implications of these limitations for the science we want to do, and I will summarise future prospects for overcoming these challenges.

Speaker: Alan McConnachie (NRC Herzberg)
• 17:35
Kinematics of The Tidally Disrupting Dwarf Galaxy Antlia II 15m

The Milky Way satellite dwarf galaxy Antlia II is one of the lowest surface brightness galaxies known. It has a size comparable to the Large Magellanic Cloud, but only 10^6 solar masses of stars. We present kinematic and chemical measurements from the Southern Stellar Stream Spectroscopic Survey using the AAT/2dF, which clearly demonstrate that Antlia II is tidally disrupting. The orbit and velocity gradient also clearly shows that the Milky Way has moved in response to the Large Magellanic Cloud. However, Antlia II currently lies on the galaxy mass-metallicity relation, suggesting that it has not lost too much stellar mass. These measurements constrain the density profile of Antlia II and generally illustrate the importance of full dynamic models when interpreting the masses of local group galaxies.

Speaker: Alexander Ji
• 20:00 22:00
Science Slam? 2h

Location: 11-LINE, Charlottenstraße 119, 14467 Potsdam

• Wednesday, 22 March
• 09:00 10:45
SESSION 3 : Dwarf galaxies: dwarf spheroidal, ultra-faint dwarfs, dwarf irregulars
Convener: Magda Arnaboldi
• 09:00
Understanding the formation of the extremely diffuse Andromeda XIX dwarf galaxy 25m

Andromeda (And) XIX is a unique dwarf galaxy in the M31 system. Its large half-light radius (in excess of 3 kpc) and low surface brightness (29.3 magnitudes per square arcsecond) make it one of the most diffuse galaxies in the known Universe. In addition to its extreme structural properties, its dynamics also suggest that it sits in a low density dark matter halo that may have been shaped by tidal forces. To understand how such a galaxy could form, we acquired deep HST imaging to resolve stars in this system down to its oldest main sequence turn-off. These allow us to measure a detailed star formation history for And XIX for the first time. In this talk, I will present our findings from these data in concert with chemodynamical observations. These will allow me to confront various theories for the formation of And XIX and discuss how such galaxies may form.

Speaker: Michelle Collins (University of Surrey)
• 09:25
An aMUSEing view of the dwarf irregular galaxy IC 1613 15m

Analysing the stellar kinematic properties of a dwarf galaxy makes it possible to investigate which internal mechanisms have shaped its evolution. Isolated dwarf galaxies with an extended star formation history, in particular, offer the opportunity to study not only star formation processes at low-mass and low-metallicity scales, but also to understand how internal kinematic properties evolve thanks to the possibility of a combined comparison of gas kinematics with that of young and old stars.

In this context, we have analysed a new spectroscopic dataset of the isolated gas-rich dwarf galaxy IC 1613 taken with the integral-field-unit MUSE instrument mounted on the Very Large Telescope (VLT), capable of combining high spatial resolution to a wide spectral coverage (4750 − 9300 AA) with a resolving power of R = 1500 − 3000. Thanks to this revolutionary instrument, we obtained a large dataset of ~2000 sources extracted from 3 pointings, of which we could reliably perform spectral classification and radial velocity determination for more than 800 stars. We were also able to obtain metallicities from Ca II triplet lines for a selected subsample of ~300 red giant branch stars. This dataset is the largest currently collected for an isolated dwarf galaxy in the Local Group.

From the spectral classification we found giant stars of all stellar types, with a large predominance of red giant branch stars, as to be expected. We were also able to identify a sample of C-star candidates, useful for determining the spectroscopic C/M ratio, and hot emission line stars as an additional kinematic tracer for young stars.
The preliminary kinematic analysis has led to a systemic velocity and a velocity dispersion both in agreement with previous literature values. We also found with high statistical significance a linear rotation signal along the optical major axis of the galaxy, which is a novelty. Stars follow the velocity field of the neutral HI component, although they start to decouple from the gas motion around the half-light radius. When subdivided according to age, young and old stars seem to follow similar kinematic trends, although the statistics for young stars drop dramatically in the outermost pointings.
Finally, chemical analysis resulted in an average [Fe/H] that was slightly higher than literature values, but within the rms dispersion of the stellar luminosity-metallicity relation. We found no evidence of a radial metallicity gradient, compatible with results from other similarly luminous dwarf galaxies.

Regardless of results actually obtained, we want to show the vast research possibilities that MUSE offers in the field of resolved stellar populations in nearby dwarf galaxies, whose surface we have only scratched.

Speaker: Salvatore Taibi (Leibniz-Institute for Astrophysics Potsdam)
• 09:40
The Dark Matter Density Profiles of Antlia B 15m

One particularly promising way to understand the nature of dark matter is to study the so-called core-cusp problem. Many solutions have been investigated to solve it and one possibility is that the nature of the dark matter itself is different from the successful $\Lambda$-Cold Dark Matter model. To reduce the impact of baryonic physics which obscures our ability to constrain dark matter, we need to study the most dark matter dominated systems known, ultra-faint dwarfs.
Here we present the first spectroscopic observations of the Antlia B, a distant (d ∼ 1.35 Mpc) ultra-faint dwarf ($M_V = -9.4$, M$_\star ∼ 8\times 10^5$M$_\odot$ ), from MUSE-Faint – a survey of ultra-faint dwarfs with the Multi Unit Spectroscopic Explorer. We measure line-of-sight velocities of 127 member stars, and combine these with GravSphere, a Jeans modelling code, to place constraints on dark matter and derive the first dark matter density profile for this object.
In particular we present constraints on the nature of self-interacting dark matter (SIDM) that is based on the assumption that the dark matter particles can scatter with one another, being able to transport heat through the dark matter halo, altering the halo structure – and, possibly, producing a constant-density core in the heart of the halo; and scalar field dark matter (SFDM) that is a Bose-Einstein condensate and a quantum superfluid and can suppress structure formation when there is a strong repulsive self-interaction. These are to our knowledge the first constraints on SFDM using ultra-faint dwarf galaxies and we show that we can rule out SFDM as an explanation for the cores in the larger dwarf galaxies in the Local Group. We also show that if Antlia B has a core produced by SFDM, the characteristic length scale of the repulsive self-interaction has to be smaller than $R_{\text{TF}} \approx 0.37$ kpc.

Speaker: Mariana P. Júlio (AIP)
• 09:55
High resolution spectroscopy of stars in the outskirts of the Tucana II ultra-faint dwarf galaxy 15m

We present kinematics and detailed chemical abundances of stars in the outskirts (out to ~8 half-light radii) of the Tucana II ultra-faint dwarf galaxy (< 10^5 Lsun; UFD) from high-resolution Magellan/MIKE spectroscopy. The Milky Way’s UFDs are “relic” galaxies (~13 Gyr old) from the early universe, making their stars unique probes of the first stages of galactic evolution. Previous spectroscopic studies had largely been limited to stars within the core of these galaxies (~2 half-light radii) due to the sparseness of their distant stars. This work presents the first high-resolution spectroscopy of a population of stars outside the core region (~4 half-light radii) of a UFD that show no obvious evidence of being unbound from the galaxy; these distant stars are not aligned with the orbital track of Tucana II and do not show evidence of a velocity gradient. The farther stars are, on average, more metal-poor than the central population, and their detailed chemical abundances provide clues to the formation of the outskirts of Tucana II. The metallicity difference between inner and outer stars suggests Tucana II, and perhaps other ultra-faints, plausibly were influenced by early, strong feedback episodes or a galactic merger as suggested by simulations (Tarumi et al. 2021). The alpha element abundances in Tucana II indicate some delayed chemical evolution, which is consistent with Tucana II being formed by an early merger of first galaxies that triggered star formation. The chemical abundances of these distant stars do not indicate that Tucana II had abnormally energetic SNe, suggesting that if SNe drove in-situ stellar halo formation then other UFDs should show similar extended features. The kinematics and location of these distant stars also hint that Tucana II may harbor a spatially extended dark matter halo (> 10^7 solar masses out to 1 kpc). Our results suggest that key factors (e.g., most metal-poor stars, evidence of extended halos) in understanding the early evolution and dynamical state of these relic galaxies lie in their outskirts and may have been missed by previous observational work. We demonstrate that detailed spectroscopic studies of individual stars in such low surface brightness features are now possible.

Speaker: Anirudh Chiti (University of Chicago)
• 10:10
Leo T Dissected with the MUSE-Faint Survey 15m

Previous studies of Ultra-Faint Dwarfs (UFDs) show that their dynamical mass-to–light ratios are the highest values measured in any type of galaxy, implying relatively pure dark-matter halos with minimal baryonic content. Furthermore, UFDs have the lowest metallicities, oldest ages, smallest sizes, and simplest assembly histories of all galaxies. Understanding the nature of these galaxies would improve immensely the knowledge of the galaxy formation process and help to unravel the nature of dark matter.

The Leo T UFD is one of the lowest mass galaxies known to contain neutral gas and to be also extremely dark-matter dominated. Previous studies have shown signs of recent star formation in the galaxy, making it one of the very faintest galaxies to show this. It is, therefore, an interesting laboratory for studies of gas and star formation at the limit of where galaxies are found to have rejuvenating episodes of star formation.

In this contribution I will discuss a novel study of Leo T that we have done using data from the MUSE integral field spectrograph. The high sensitivity of MUSE allowed us to obtain velocity measurements for stars as faint as magnitude ~24, which allowed us to increase the number of Leo T stars observed spectroscopically from 19 to 75.
Combining the MUSE data with photometric data from HST, we have studied the age and metallicity of these stars and identified two populations, all consistent with similar metallicity. Within the young population sample we discovered three emission line Be stars - a first for ultra-faint dwarfs.
While looking for differences in the dynamics of young and old stars, we find that they have different kinematics, with the young population having a velocity dispersion consistent with the kinematics of the cold component of the neutral gas.
In this contribution I will discuss these results and their implications for the origin and evolution of Leo T.

Speaker: Daniel Vaz (Instituto de Astrofísica e Ciências do Espaço)
• 10:25
3D Velocities of Local Group dwarf galaxies combining HST and Gaia 2m

Gaia has revolutionized our understanding of the Milky Way (MW) and its satellite system. However, the proper motions (PMs) of dwarf galaxies outside the MW system remain out of reach by Gaia and will remain so even with future data releases. This is problematic, as it means that our understanding of systems outside the MW will be limited and we may be basing much of our cosmological modeling on the MW alone, which may be atypical. This limitation can be overcome through the use data from the Hubble Space Telescope, either obtained at two different epochs, or at one past epoch and combined with more recent Gaia positions. I will present new PMs obtained in this manner for a sample of distant dwarf galaxies in the Local Group (LG), expanding the number of systems for which we have full position-velocity information. These can be used to calculate the implied orbits. I will discuss how this information allows new science with the LG dwarfs and their hosts, linking the star formation and orbital histories of dwarfs to determine when specific dwarfs quenched and whether they are first-infall or ‘backsplash’ galaxies and how we can use this to determine host properties such as halo mass and the extent of the hot gas halo.

Speaker: Paul Bennet (Space Telescope Science Institute)
• 10:27
Stellar metallicities in the extremely low mass galaxy Leo P 2m

Leo P is a poorly studied near-primordial isolated dwarf irregular galaxy at a distance of ∼ 1.6 Mpc, with an extremely low mass of ∼ 105M⊙.
From Hubble Space Telescope and Arecibo Legacy Fast observations, its characteristics show the same behaviour as expected in a low-luminosity dSph Milky Way satellite. It was defined as the “quintessential system to test theories of how the smallest structures in our universe survive and grow.”
Observations suggest an ongoing star formation in its prominent H II region. This feature also gives estimations of the oxygen abundance which is around 3% of solar. It makes it one of the most metal-poor galaxies known in the local Universe.
In this work, we present the first estimated spectroscopic metallicities of evolved cool stars in Leo P. We use MUSE data to extract and analyse the spectra of 16 red giant branch stars. In order to derive the metallicity of Leo P we apply literature calibrations for our equivalent widths measurements of the Ca II triplet lines.
We will also present tentative luminosity- and stellar mass-metallicity relations compared to other low-metallicity galaxies (e.g. I Zw 18, DDO 68, and SBS 0335-052W) to check the previously detected offset of these other galaxies, with respect to Leo P, in these relations.

Speaker: Heitor Ernandes (Lund Observatory)
• 10:29
Kinematical asymmetry in the dwarf irregular galaxy WLM and a perturbed halo potential 2m

To investigate the dynamical nature of the kinematic asymmetry in the isolated gas-rich dwarf irregular galaxy W LM in the Local Group, we first examine whether an m = 1 perturbation in the halo potential could be a mechanism creating such kinematical asymmetry.
By fitting a theoretical rotational velocity associated with an m = 1 perturbation in the halo potential model to the observed data, we show that such a lopsided halo potential model can explain the asymmetry in the kinematic data reasonably well.
In addition, we study the kinematical classification of the velocity field of WLM with various methods, and based on a kinemetry analysis, we find that it is possible for WLM to lie in the transition region, where the disk and merger coexist. Thus, a merger may indeed be one of the possible origins of the dark matter halo lopsidedness for this isolated galaxy.

Speaker: Maryam Khademi (IPM , Institute for Research in Fundamental Sciences (IPM), School of Astronomy)
• 10:31
The Cosmic Hunt for Members in the Outskirts of Ultra Faint Dwarf Galaxies: Ursa Major I, Coma Berenices, and Boötes I 2m

The chemical abundance patterns exhibited by stars in Ultra-Faint Dwarf (UFD) galaxies can provide a wealth of information about the evolutionary history of UFDs, including what dynamical history the stars in a UFD may have experienced. Of particular interest are stars in the outer regions of UFDs because they may hold evidence of tidal stripping, mergers, or other mechanisms; and because of the localized nature of enrichment events in UFDs, it is important to use every available star in analyses of UFD histories. I present detailed abundances from high-resolution spectroscopy with Gemini/GRACES for five new stars in three UFDs, Coma Berenices (Com Ber), Ursa Major I (UMaI), and Boötes I (BooI), where four of our five stars are at distances greater than two half-light radii. The chemistries for all three galaxies are consistent with the outermost stars forming in the central regions, then moving to their current locations through dynamical mechanisms. In BooI, the lower metallicity and lack of strong carbon enrichment of its outermost stars could also be evidence of a dwarf galaxy merger. The abundance ratios and chemical patterns of the stars in Com Ber are consistent with contributions from SN Ia (including an unusually high Ni value), which is unexpected for its star formation history and in conflict with previous suggestions that this system evolved chemically from a single core collapse supernova event. We look forward to the new GHOST (Gemini High-resolution Optical SpecTrograph), which will make these types of chemical abundance-quality observations possible for many more stars in UFDs.

Speaker: Fletcher Waller (University of Victoria)
• 10:33
Dynamical evolution of the sub-galactic dark matter halos and the effect of the cusp-to-core transformation 2m

In the hierarchical clustering scenario based on the $\Lambda$ cold dark matter ($\Lambda$CDM) model, sub-galactic dark matter halos (subhalos) are of crucial importance in building up the larger structures via merging processes. Moreover, dwarf galaxies around the Milky Way (MW) are ideal sites for studying the nature of dark matter since they are dark-matter-dominated systems.

We investigate the mass evolution histories and the evolutional tracks on the $r_\mathrm{max}$-$V_\mathrm{max}$ plane of the subhalos associated with MW-sized host halos based on the ultra-high resolution cosmological $N$-body simulation, Phi-4096 by Ishiyama et al. (2021), where $V_\mathrm{max}$ and $r_\mathrm{max}$ denote the maximum circular velocity and the radii where the circular velocity becomes $V_\mathrm{max}$, respectively. We report that the evolutional tracks of dwarf-galaxy-mass halos show an interesting feature compared to the ones of massive halos shown in the literature. The mass evolution of subhalos clearly shows two phases: the accretion phase and the stripping phase. First, more than 90% of our sample subhalos increase their masses gradually until the redshift $z=1-2$ (accretion phase), and then the masses decrease due to the tidal stripping driven by the host halo (stripping phase). Thus, we show quantitatively that tidal stripping plays an essential role in the dynamical evolution of subhalos.

In addition, the mass density profile of subhalos has a long-standing disagreement between the observations and the predictions by the pure CDM simulations, known as the cusp-core problem. Standing on a hypothesis that DM halos are formed primarily with central cusps and then some dynamical processes lead them to form cores, we provide a ‘core-to-cusp transformation model’ that reverts the properties of cored profile to ones of the cusp profile. In constructing a model, some appropriate conditions are imposed about how the transition should occur, and detailed physical processes are not considered here. The reverted observational properties, including $r_\mathrm{max}$ and $V_\mathrm{max}$, show excellent agreement with the theoretical prediction from the cosmological $N$-body simulation. This result supports the description that the CDM creates initial cusps in the centre of DM halos, and some dynamical process gives rise to the cusp-to-core transformation.

Speaker: Yuka Kaneda (University of Tsukuba)
• 10:35
Tidally perturbed 'feeble giant' dwarf galaxies 2m

The unusually low velocity dispersion and large sizes of feeble giant' galaxies, such as Crater II or Antlia II, pose a challenge to our understanding of dwarf galaxies in the Lambda Cold Dark Matter (LCDM) cosmogony. Their low velocity dispersions suggest either a dark halo mass much lower than the minimum expected from hydrogen cooling limit arguments, or one that is in the late stages of extreme tidal stripping. The tidal interpretation has been favoured in recent work and is supported by the small pericentric distances consistent with available kinematic estimates from Gaia. We use N-body simulations to examine this interpretation in detail, assuming a Navarro-Frenk-White (NFW) profile for the Crater II and Antlia II progenitor halos. Our main finding is that, although the low velocity dispersions can indeed result from the effect of tides, the large sizes of feeble giants are inconsistent with this hypothesis. This is because galaxies stripped in mass to match the observed velocity dispersions are also reduced to sizes much smaller than the observed half-light radii of Crater II and Antlia II. Unless their sizes has been substantially overestimated, reconciling systems like these (including Andromeda XXV and XIX) with LCDM requires that either (i) they are not bound and near equilibrium (unlikely, given their crossing times are shorter than the time elapsed since pericentre), or that (ii) their progenitor halos deviate from the assumed NFW profile. The latter alternative may signal that baryons can affect the inner halo cusp even in extremely faint dwarfs or, more intriguingly, may signal effects associated with the intimate nature of the dark matter, such as finite self-interactions, or other such deviations from the canonical LCDM paradigm.

Speaker: Alexandra Borukhovetskaya (University of Victoria)
• 10:37
Census of Northern Local Group Dwarf Galaxies using UNIONS 2m

Dwarf galaxies play a key role in probing the dynamics of the Milky Way, the history of star formation in the Local Group, and the accretion and retention of gas in dark matter halos at the smallest scales of galaxy formation. In this work, we use the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS), the deepest, widest photometric survey ever carried out in the northern hemisphere, to complete a systematic analysis of 3600 sq. deg. of sky in search of new dwarf galaxies within 1 Mpc of the Milky Way. This search aims to detect Milky Way satellites, M31 satellites, and nearby, isolated dwarf galaxies using a matched-filter algorithm. To date, all previously known dwarf galaxies in the survey footprint have been recovered, each with high statistical significance. We present the discovery of a new Milky Way dwarf galaxy candidate, a new, faint star cluster, and report on the follow-up of numerous high confidence candidate detections. The end goal of this work will be to assess the completeness of the Milky Way satellite population as a function of position, luminosity, mass, and size. UNIONS imaging, when completed, is the approximate depth of the LSST after a year of operation, and our studies of the northern hemisphere provide a complementary approach and foretaste of what will come in the era of Rubin.

Speaker: Simon Smith (University of Victoria)
• 10:45 11:30
Poster Session & Coffee Break 45m
• 11:30 13:00
SESSION 3 : Dwarf galaxies: dwarf spheroidal, ultra-faint dwarfs, dwarf irregulars
Convener: Magda Arnaboldi
• 11:30
Revealing mass distributions of dwarf spheroidal galaxies in the Subaru-PFS era 15m

The dwarf spheroidal galaxies (dSphs) in the Local Group are excellent test beds for probing the properties of dark matter and its role in galaxy formation.
These galaxies are sufficiently close that it is possible to measure line-of-sight velocities for large samples of resolved stars. This kinematic information enables us to study the structural properties of their dark matter halos.
However, there are non-negligible uncertainties in the determination of the mass profiles of dark matter.
In particular, this study has been hampered by the well-known degeneracy between dark matter mass density and the anisotropy of the stellar velocity dispersion tensor, which can lead to erroneous mass estimates.
The information encoded in the shape of the line-of-sight velocity distribution (LOSVD) is potentially a strong tool to break this degeneracy, but this requires sufficiently large kinematic samples over the full radial extent of dSphs and identification of foreground contamination.
The combination of the 1.25 deg^2 field and 2394 fibers of the Subaru Prime Focus Spectrograph (PFS), plus pre-imaging with Hyper Suprime Cam, will allow us to make significant progress in this undertaking. Furthermore, the unique capability of PFS will permit us to revisit the core/cusp problem.
In this talk, I will discuss the feasibility and future prospects for this dark matter study with Subaru-PFS survey using mock stellar samples.

Speaker: Kohei Hayashi (NIT, Ichinoseki college)
• 11:45
Small-scale stellar haloes: detecting extended substructure in the outskirts of Milky Way dwarf galaxy satellites 15m

Dwarf galaxies are valuable laboratories for dynamical studies related to dark matter and galaxy evolution, yet it is currently unknown just how extended their stellar components are. Each satellite orbiting within the Milky Way’s (MW’s) gravitational potential may undergo tidal stripping by the host galaxy, or alternatively, may themselves have accreted yet smaller systems whose debris settles into the satellite’s own stellar halo. Both processes could mean that significant populations of member stars are found far from the center of the dwarf. Stars in the outskirts of these systems are especially valuable – and rare – tracers of the dwarf’s dynamics in low acceleration regimes, and they give insight into the dwarf’s evolutionary history. In this work, we examine the MW’s ~60 dwarf satellites to search for these rare, distant member stars. Using Gaia eDR3 and a maximum likelihood approach allowing for multi-component extended substructures, we find 9 dwarfs that exhibit a secondary, lower-density, outer profile, that we argue is indicative of an extended stellar halo and/or tidal disruption. Our method shows excellent consistency with spectroscopically confirmed members from the literature and requires no radial velocity information. For each dwarf galaxy, we derive a sample of high-confidence members which will prove useful for studying even the faintest MW dwarfs. We also briefly discuss a current spectroscopic follow-up campaign for the most radially distant outskirt members, which will feature the newly commissioned Gemini High-resolution Optical SpecTrograph (GHOST). Already, initial spectra obtained during GHOST commissioning has proven useful for comprehensive chemodynamics of these Galactic building blocks.

Speaker: Jaclyn Jensen (University of Victoria)
• 12:00
Gaia EDR3 proper motions, energies, angular momenta of Milky Way dwarfs: a recent infall to the Milky Way halo 25m

Gaia EDR3 has provided proper motions of Milky Way (MW) dwarf galaxies with an unprecedented accuracy, which allows us to investigate their orbital properties. We found that the total energy and angular momentum of MW dwarfs are much larger than that of MW K-giant stars, Sagittarius stream stars and globular clusters. It suggests that many MW dwarfs have recently infall into the Milky Way halo. We also confirmed that Milky Way dwarfs lie preferentially near their pericenters and many of them belong to a vast polar structure perpendicular to the Milky Way disk, which suggests that Milky Way dwarfs do not behave like satellite systems derived from LCDM cosmological simulations. These new results require revisiting the origin of MW dwarf galaxies, e.g., if they came recently, they were likely to have experienced gas removal due to the ram pressure induced by MW’s hot gas, and to be affected by MW tides. We will discuss the consequences of these processes on their mass estimation.

Speaker: Yanbin Yang (CNRS France / Paris Observatory)
• 12:25
Lopsidedness in Andromeda's satellite galaxy distribution: A renewed tension with CDM simulations 15m

Dwarf satellite galaxies around Andromeda (M31) and the Milky Way form thin, coherently rotating planes argued to be in tension with expectations from the cold-dark-matter (CDM) model of cosmology. For M31, this disagreement is compounded by a prominent asymmetry in its satellite distribution; over 80% of its dwarfs lie in the hemisphere facing the Milky Way. To a degree, lopsided dwarf galaxy systems appear to be ubiquitous in the local Universe and may reflect an underlying asymmetry in their hosts' dark matter halo morphology - in turn carrying implications for halo mass estimates derived from satellite dynamics. Adopting a recently published set of homogeneous, RR Lyrae-based distances to the M31 satellites, we discovered that the existing asymmetry is strengthened. 34 out of 35 satellites are contained within a cone with an opening angle of 202 degrees (or 213 degrees facing the Milky Way), while the luminous dwarf M110 dominates the nearly hemispheric void on the other side. We further studied the rarity of similarly asymmetric dwarf galaxy distributions in several state-of-the-art cosmological simulations. Even when accounting for the look-elsewhere effect in selecting a preferred opening angle, less than 0.4% of M31 analogs host satellite systems that match or exceed the observed asymmetry. The significance of the M31 satellites' observed asymmetry towards the Milky Way in CDM simulations now rivals that of M31's plane-of-satellites, cementing the Andromeda system as a striking outlier from cosmological expectations.

Speaker: Kosuke Jamie Kanehisa (Leibniz-Institut für Astrophysik Potsdam)
• 12:40
Does the presence of old globular clusters in low-mass galaxies disprove modified gravity? 2m

The discussion "dark matter vs. modified gravity" has not been resolved yet. It was proposed that dynamical friction could be used to discriminate between the two alternatives. Analytic calculations indicate that, with modified gravity, globular clusters (GCs) of low-mass galaxies experience much stronger dynamical friction than in the equivalent system with Newtonian gravity and dark matter. As a result, in modified gravity the dynamical friction should have already forced the old GCs of low mass galaxies to settle in the centers of the galaxies. This is not observed. I will report on our efforts to verify the analytic results by self-consistent simulations with the MOND gravity. We published already that the core stalling mechanism, that was not considered in the analytic calculations, prevents GC to settle in centers of ultra-diffuse galaxies. In our ongoing work, we investigate GCs of isolated dwarf galaxies. It seems so far that GCs of these galaxies survive if the do not move within the galaxy disk, or if they counterrotate with respect of the galaxy disk. In the simulations done so far, supernova explosions influence the orbits of GCs only at low numerical resolution.

Speaker: Michal Bilek
• 12:42
Environmental secular evolution of satellite galaxies 2m

In this talk I will present our results on environmental secular evolution processes that affect satellite galaxies as they enter their hosts.
Our approaches consist of global statistical analysis of satellites, and the modelling of detailed observations. For the latter approach we study distant gas rich dwarf satellites like Leo T and Phoenix, which are entering the Milky Way. Both satellites present non-equilibrium offsets between their gaseous and stellar distributions, with Leo T also showing an offset between its younger and older stellar populations. Using hydrodynamical simulations that include the Milky Way coronal wind, we find that cored dark matter density models can better reproduce the estimated timescales of the offsets in Leo T.
From the global approach we present our latest analysis of the Milky Way and the Andromeda satellite galaxies, finding a transition radius at R*~0.4-0.6Rvir that delimits an inner satellite population with stellar densities that correlate with the tidal field of their hosts, and an outer less processed population. Furthermore, we find that this transition radius is also present in satellites of the Fornax and Virgo galaxy clusters, as well as in their cosmological galaxy simulation counterparts.

Speaker: Matias Blaña (Instituto of Astrophysics, Pontificia Universidad Católica de Chile)
• 12:44
Quenching and ram pressure in simulated low-mass Local Group galaxies 2m

Low-mass or dwarf galaxies are particularly compelling laboratories for star formation quenching because they are highly susceptible to quenching effects from both internal stellar feedback and external environment. We explore the role of ram pressure in the environmental regulation of gas content and quenching of low-mass galaxies in zoom-in hydrodynamic simulations of Milky Way (MW) mass hosts. The quiescent fraction of low-mass galaxies increases as their stellar mass decreases and as their distance to a MW-mass host decreases, similar to the Local Group. In addition, the location of a satellite at quenching (in a MW halo, in a low-mass group, or in isolation) and the timing of quenching with respect to different events like infall or pericenter passage depend on the mass of the satellite galaxy. In particular, we find that MW satellites can be efficiently quenched before infall into a MW halo by pre-processing in low-mass groups, where they experience ram pressure comparable to that in a MW halo. Interestingly, the density of halo gas near paired Local Group-like hosts is enhanced at small angles/latitudes off the host galaxy disk versus directly above or below the disk. Preliminary results indicate that both observed and simulated satellites within these low-latitude regions at z=0 may be preferentially quenched, similar to recently reported anisotropic quenching in massive galaxy clusters at low redshift. In addition, we briefly discuss morphological changes to low-mass galaxies induced by the Local Group environment, such as the formation of ultra diffuse galaxies via tidal shocks.

Speaker: Jenna Samuel (UT Austin)
• 12:46
The origin of stellar ellipticity in tidally isolated ultra-faint dwarfs 2m

Ultra-faint dwarf galaxies (UFDs) are often found in large numbers in close proximity to the Milky Way and other massive spiral galaxies. As such, their projected stellar ellipticity and extended light distributions are often thought to owe to tidal forces. I discuss the projected stellar ellipticities and faint stellar outskirts of isolated ultra-faints, drawn from the Engineering Dwarfs at Galaxy Formation’s Edge’ (EDGE) cosmological simulation suite. I find that, in spite of their tidal isolation, the simulated dwarfs exhibit a wide range of projected ellipticities ($0.03 < \varepsilon < 0.75$), and show a strong correlation between their ellipticity and formation time. Furthermore, many have anisotropic extended stellar outskirts (the furthest in EDGE being out to 143 half light radii) that can masquerade as tidal tails but are actually due to mergers. I show that the distribution of projected ellipticities in the sample of simulated EDGE dwarfs matches very well that of 22 tidally isolated Local Group dwarf galaxies. These results imply that a significant number of UFDs found to date are tidally isolated, as further suggested by their large orbital peri and apocentres. We argue that this tidal isolation makes nearby UFDs excellent natural laboratories for testing galaxy formation and dark matter models.

Speaker: Alex Goater (University of Surrey)
• 12:48
The Formation of Planes of Satellite Galaxies in the Local Group 2m

Numerous observations in recent years have shown that the satellite galaxies orbiting our local galaxies tend to align their orbits in one or two thin planes around the host galaxy. This has been observed in local galaxies, Andromeda and Centaurus A, and our own Milky Way. Numerical simulations in a cosmological context find these planes to be rare or short-lived leading to tension between observation and theory. This leads to considerable debate on whether observations are compatible with the standard, Lambda Cold Dark Matter, model of cosmology. We argue that on large scales, these simulations did not sufficiently resolve the nearby large-scale structure, cosmic filaments, which we believe to be responsible for the anisotropic infall of satellites forming planar alignments, and on smaller scales, they did not sufficiently resolve dwarf satellite galaxies. We use the high precision, hydrodynamic, cosmological zoom simulation, New Horizon, which has both the large volume, (16 Mpc)$^3$ and the small-scale resolution, ~ 35 pc, required to study the interplay between cosmic web dynamics and the formation, funneling, and eventually the anisotropic distribution of satellites around local galaxies. Our results indicate that these planes exist in New Horizon in ~ 30% of Milky Way-type systems. The identified planes are comparable to observation in both physical extent and kinematic coherence. We also find that the distribution of dwarf satellites within their host dark matter haloes is more anisotropic than previously understood.

• 12:50
Dynamical modelling of cosmologically simulated Dwarf galaxy 2m

Prolate rotation in galaxies (rotation around the major axis) is a rare phenomenon in the Universe. The effect has been exclusively attributed to past major mergers and thus studies of prolate-rotating systems can help us better understand the hierarchical process of galaxy evolution. Dynamical studies of such galaxies is important to find their gravitational potential profile, total mass, and dark matter fraction. Recently, it has been shown from cosmological simulation that it is possible to form a prolate-rotating dwarf galaxy following a dwarf-dwarf merger event. The simulation also shows that the unusual prolate rotation can be time enduring. In this particular example the galaxy started rotating around its major axis about 7.4 Gyr ago and it is still continuing at the present time. In this project, we use mock observations of the hydrodynamically simulated galaxy to fit various stages of its evolution with Jeans dynamical models. The Jeans model successfully fits the early oblate state and also for late prolate stage of simulated galaxy, prior the major merger event, recovering the anisotropy, mass density distribution, velocity dispersion, and rotation of the simulated galaxy. This master thesis project is an important connecting link between cosmological simulations and real observational data, as many prolate rotation galaxies are being discovered.

Speaker: Amrit Sedain (University of Potsdam)
• 13:00 13:15
Conference Photo 15m
• 13:15 13:30
Poster Session 15m
• 13:15 14:00
Tour Telegrafenberg OR visit Great Refraktor 45m
• 13:30 14:30
Lunch Break 1h
• 14:30 15:30
Plenary Discussion 3 1h
Speakers: Elena Sacchi (AIP), Matthew Walker (Carnegie Mellon University)
• 15:30 16:05
SESSION 4 : The Magellanic System: the Clouds, the Stream and the Leading Arm
Convener: Oliver Müller
• 15:30
Exploring the kinematics of the Magellanic stellar periphery 25m

Recent panoramic maps of the Magellanic system have revealed a wealth of low-surface-brightness stellar substructures surrounding both the Large and Small Magellanic Clouds (LMC/SMC); clear evidence of tidal interactions between the two Clouds, as well as with the Milky Way. However, the interaction history of the Magellanic system beyond the most recent LMC/SMC close passage remains poorly constrained. In order to shed light on this issue, we have instigated a large-scale spectroscopic follow-up of stars in low-density features extending to distances beyond 20 degrees from the Clouds’ centres. We use a combination of Gaia astrometry and spectroscopically-derived radial velocities, obtained with 2dF+AAOmega on the Anglo-Australian Telescope, to determine 3D kinematics for thousands of stars in these features and the extended outer disks of the two Clouds. In this talk, I will discuss new results focussed on the southern outskirts of the LMC. Several substructures in this region, including claw-like features extending from the southern LMC disk, and a long arm-like substructure wrapping around the southern LMC outskirts toward the eastern SMC disk, are found to be predominantly composed of perturbed LMC disk material. All substructures show significant perturbations from equilibrium disk kinematics, with one claw-like feature displaying out-of-plane velocities exceeding 60 km/s and apparent counter-rotation relative to the LMC’s disk. Such complex features plausibly require multiple previous interactions with the SMC to fully explain the observed dynamical properties. This demonstrates the efficacy of our data as a benchmark for assessing dynamical models to disentangle the origins of Magellanic substructures, the masses of the two Clouds, and the evolution of the Magellanic system. I will also briefly discuss new efforts to conduct analogous kinematic mapping of M33 and its outskirts, which aim to similarly understand the evolution of this massive dwarf galaxy.

Speaker: Lara Cullinane (JHU)
• 15:55
Ages, metallicities and structure of stellar clusters in the Magellanic Bridge 2m

The Magellanic Bridge is a tidally-stripped structure located between the Magellanic Clouds and contains hundreds of stellar clusters and associations, which can help understanding the origin and evolution of the entire Magellanic Clouds-Milky Way (MW) system. Two main competing models describe the formation of the pair LMC-SMC: the LMC captured the SMC about 2 Gyr ago and they are in a bound orbit around the MW, or it is an old interacting sys- tem in its first perigalactic passage, falling into the MW potential ~2 Gyr ago. The Bridge should have been formed during a collision between the Clouds around 200 Myr ago, imply- ing kinematic signatures, as well as age and metallicity gradients along its extension. This work combines deep photometric data from VISCACHA and SMASH surveys in order to ex- plore this question, by homogeneously deriving age, metallicity, distance, structural parame- ters and mass of 35 Bridge objects with modern statistical tools such as Markov chain Monte Carlo and machine learning. In particular, the mass determination corrected by completeness can help to estimate the Bridge stellar mass and constrain the dynamical models. A spectro- scopic follow-up in the CaII triplet region is also carried out for clusters older than 1 Gyr in the Bridge and other SMC regions, to derive metallicity and radial velocity which, combined with Gaia proper motions, allow us to obtain a 6D phase-space vector. Preliminary results show a good agreement between VISCACHA and SMASH data and also that, despite the presence of some old objects at larger distances, the clusters in the middle of the Bridge are younger (~ 10-100 Myr) and more metal-rich than those closer to the SMC Wing. In this poster, we will review the recent results of the VISCACHA collaboration, present a detailed approach to age and metallicity gradients in the Bridge, and discuss the evidence on the most probable for- mation model of the pair LMC-SMC.

Speaker: Raphael A. P. Oliveira (IAG - USP)
• 15:57
Stromgren photometric metallicity map of the Small Magellanic Cloud using Gaia DR3-XP spectra. 2m

Observational studies have identified several sub-structures in different regions of the Magellanic Clouds (MCs). One such interesting sub-structure in the Small Magellanic Cloud (SMC) is a dual population of intermediate-age giant stars which are spatially and kinematically distinct. Comparisons with simulations suggest that the foreground population might be tidally stripped from the SMC main-body, but their origin is not clearly proven yet. If we have homogeneous metallicity measurements of the sources from these sub-structures, we can see whether they are having similar values or different from the main-body population. Hence, metallicity measurements of these populations will help us understand their origin and/or their association with the Large Magellanic Cloud (LMC). However, spectroscopic metallicities are only available for a few thousand sources and also from different instruments at various spectral resolutions, which makes it difficult to compare their values and draw conclusions from the results. The third data release of Gaia has provided us with ~ 0.17 million XP spectra of the SMC sources as faint as ~ 18 mag in G-band which are spread across ~ 10° from the SMC center. Stromgren photometry is a well-established method to estimate the photometric metallicities. Gaia BP spectra covers the u, v, b and y Strömgren bands. Using the estimated Strömgren magnitudes from the Gaia BP spectra, we calibrated the [Fe/H] values. We compared a subset of the SMC sources that has [Fe/H] values from the high-resolution (~ 22,500) APOGEE spectrograph for the validation of our method. Using those metallicity measurements, we produce a homogeneous metallicity map of the entire SMC also with a higher spatial resolution and study the metallicity of different sub-structures to shed light on their possible origin and/or their associations with the LMC.

Speaker: Abinaya Ondivillu Omkumar (Leibniz Institute for Astrophysics Potsdam (AIP))
• 15:59
Spatio- temporal formation history and kinematics of star clusters in the Magellanic Clouds: Propagation of cluster formation and the LMC bar 2m

The Large Magellanic cloud(LMC) and Small Magellanic cloud(SMC) are the nearest interacting dwarf galaxies in the local group. And Magellanic clouds(MCs) have had interactions with each other as well as with the Milky Way. These interactions have triggered star formation in both galaxies, resulting in the formation of star clusters. Thus a comprehensive analysis of cluster population in MCs could indicate the structural evolution due to galaxy mergers. Our aim is to understand the age dating, spatio-temporal map of cluster formation episodes, and trace the cluster kinematics as a function of age.
In this study, we used the GAIA DR3 data to characterise the cluster population in the MCs. The clusters in the MCs is obtained mostly from general catalogue given by Bica et al. (2020), and several other recent literature. We classified the clusters as isolated and merged based on projected sky coordinates. Around 2000 clusters were analyzed in our study based on a certain selection criteria. We used Gaia asynchronous query to bulk handle the cluster data from Gaia archive, after applying proper motion-parallax cut offs and faintness magnitude limit. We developed a statistical algorithm to remove filed stars from the cluster region. We used single to multiple comparison field regions and retrieved the most probable cluster members. Using Padova-Parsec stellar evolutionary models, we performed an iterative least square method to estimate the age and reddening of clusters that had sufficient members and a satisfactory fit of the isochrone. The spatio-temporal map of age and reddening was obtained for clusters in both clouds. We traced the cluster formation episodes at ~ 1.6 Gyr, 630 Myr, 310 Myr, and 80 Myr. The directional propagation of cluster formation is noted from the South to North of LMC over the time scale of 2.8 Gyr to 10 Myr. We are able to estimate the formation time of the LMC’s off-centered bar. Using the proper motion data of the members we also study the proper motion of the clusters with respect to the field population in both the Clouds.

Speaker: Dhanush Sathiavals Reselammal (Senior Research fellow, IIA)
• 16:01
Tracing the structure and history of the Small Magellanic Cloud using CaII Triplet spectroscopy 2m

The Small Magellanic Cloud (SMC), as one of the nearest galaxies to us, provides a superb laboratory for studying its stellar populations in exquisite detail. We collected the largest sample of SMC red giant branch (RGB) stars (~6000) observed using the AAOmega spectrograph fed by the Two Degree Field (2dF) multi-object system at the Anglo-Australian Telescope of the Siding Spring Observatory (Australia). The metallicities were recovered using a direct estimation of [Fe/H] from the equivalent widths of the Calcium triplet (CaT). We discuss the potential implications of the metallicity gradients and compare these to previous determinations of star formation histories to assess the consequences of encounters between the SMC and Large Magellanic Cloud.
Finally, we discuss our findings in the context of the correlation that exists between metallicity and dynamical mass in dwarf Irregular systems, of which the SMC is our closest example.

Speaker: Mahdieh Navabi (The University of Surrey)
• 16:03
Modeling Dark Matter Halo Spin using Observations and Simulations: application to UGC 5288 2m

Dark matter halo properties are well studied in cosmological simulations but are very challenging to estimate from observations. The dark matter halo density profile of galaxies from observations has been modeled previously using multiple probes that trace the dark matter potential, however, the angular momentum distribution of the dark matter halos is still a subject of debate. In this study, we demonstrate a method for estimating the halo spin and halo concentration of a low luminosity, gas-rich dwarf galaxy by forward modeling disk properties derived from observations of the stellar and gas surface densities, the disk scale length, the neutral hydrogen rotation curve, the bar length, and bar ellipticity. Our method is a combination of semi-analytical techniques and N-body/SPH simulations. Here, we apply our method to the low surface brightness (LSB) dwarf galaxy UGC 5288, and model its dark matter halo with both a cuspy Hernquist profile and a flat-core pseudo-isothermal profile. We find that the best match with observations is a pseudo-isothermal halo model with core radius r$_{c}$ = 0.23 kpc and a high halo spin $\lambda$ = 0.08 at the virial radius. These findings are consistent with previous rotation curve estimates of the halo density profile of UGC 5288, as well as the theoretically predicted high spins of dwarf LSB galaxies. We finally compare our results with the halo spin distribution of barred galaxies in one of the high resolution cosmological magneto-hydrodynamical simulations TNG-50. We find that our model predicts halo spin distribution of UGC5288 up to a ballpark value, although, there remain significant uncertainties due to the formation history of the dark matter halos from the TNG50 simulations. We also find that the inner halo spin in barred galaxies is different from that of unbarred ones, and the halo spin shows weak correlations with bar properties.

Speaker: Ansar Sioree (Indian Institute of Astrophysics, Bengaluru)
• 16:05 16:50
Poster Session & Coffee Break 45m
• 16:50 18:00
SESSION 4 : The Magellanic System: the Clouds, the Stream and the Leading Arm
Convener: Oliver Müller
• 16:50
The Magellanic System 40m

The Magellanic Stream is the most spectacular example of a gaseous stream in the local Universe. In this review I will discuss the Stream's importance for many areas of Galactic astronomy, summarize key unanswered questions, and identify future observations and simulations needed to resolve them.

Speaker: Elena d’Onghia (UNIVERSITY of WISCONSIN–MADISON)
• 17:30
The Mass of the Large Magellanic Cloud 15m

The Large Magellanic Cloud (LMC) is the largest of the dwarf galaxies orbiting the Milky Way (MW). It sits in a very interesting niche within the Local Group (LG), being both sufficiently different in mass than the MW to be an interesting comparison and sufficiently massive to be a major player in the MW's recent history and present state. In particular, it is massive enough to have a significant and observable effect on the MW itself and on stellar streams in the MW halo, though the extent of its influence depends on its mass. Its mass is also interesting as a benchmark against which we can interpret observations of more distant objects.

Bennet et al. (2022) recently measured proper motions (PMs) -- using HST, Gaia or both HST and Gaia together -- for a set of globular clusters (GCs) in the LMC. Supplemented with literature distances and line-of-sight velocities, this provides a catalogue of 6D phase space information for 32 LMC GCs. These are ideal dynamical tracers of the LMC’s potential.

In my talk, I will describe how we have used this set of tracers to estimate the anisotropy and mass of the LMC within 13 kpc, and then how we have used these estimates to extrapolate the LMC’s virial mass. This is the first time that this family of mass estimation methods has been applied to the LMC, and I will also compare our estimate against other estimates of the LMC’s mass via different methods and discuss the broader context of our results.

Speaker: Laura Watkins (AURA for ESA, STScI)
• 17:45
The Formation of Magellanic System and the total mass of Large Magellanic Cloud 15m

The Magellanic Stream is unique to sample the MW potential from ~50kpc to 300
kpc, and is also unique in constraining the LMC mass, an increasingly important
question for the Local Group/Milky Way modeling. I will compare on the
strengths and weaknesses of the two types of models (tidal and ram-pressure) of
the Magellanic Stream formation. I will present our modeling for the formation
of the Magellanic System, including those of the most recent discoveries in the
Stream, in the Bridge and at the outskirts of Magellanic Clouds. This model has
been successful in predicting most recent observations in both properties of
stellar and gas phase. It appears that it is an over-constrained model and
provides a good path to investigate the Stream properties. In particular, this
model requires LMC mass significantly smaller than 10^11 Msun.

Speaker: Jianling WANG (National Astronomical Observatories, Chinese Academy of Sciences)
• Thursday, 23 March
• 09:00 10:30
SESSION 4 : The Magellanic System: the Clouds, the Stream and the Leading Arm
Convener: Ekta Patel
• 09:00
The OC stream's evolution in the dark matter haloes of the Milky Way and the LMC 25m

The OC stream is a dwarf galaxy stream and is one of the longest and best-measured streams in the Galaxy, spanning over 200 degrees on the sky. It extends from the inner Milky Way ($\sim$15 kpc) to the outer halo ($\sim$60 kpc), giving us a great tool with which to measure our Galaxy's dark matter halo. In addition, portions of the stream pass remarkably close to the LMC ($\sim$5 kpc), allowing us to simultaneously measure the properties of our Galaxy and the LMC. By combining data from Gaia DR3 with the S5 survey, LAMOST, SDSS, and APOGEE, I will present a 6D view of the stream. In light of this powerful data, we fit the OC stream using a flexible model of the Milky Way halo and the LMC. In particular, we measure the Milky Way's mass to a precision of 4% in the middle of the stream's radial extent, $\sim$2.85$\times10^{11}M_\odot$ at $\sim$32.4 kpc. We also infer a highly flattened dark matter halo of the Milky Way, with the data preferring an oblate (q = $\sim$0.55) and prolate (q = $\sim$1.40) halo over a spherical one. Interestingly, we find that both of these haloes are producing a similar forcefield in the orbital plane of the OC stream, suggesting they may be attempting to mimic a forcefield which cannot be described with by a flattened halo. We also measure the LMC's dark matter halo. In particular, we find the LMC has a mass of $\sim1.3\times10^{11} M_\odot$, which is one-sixth the Milky Way's virial mass. Our fits also suggest that the LMC's dark matter halo must extend out to at least $\sim$53 kpc from the LMC, consistent with the LMC being on first approach to the Milky Way. Finally, we find that the OC stream's close passage with the LMC occurs $\sim$300 Myr ago. Since the OC stream is very sensitive to the LMC's location at this time, this allows us to better constrain the LMC's past orbit up to that time. For the first time, we constrain the amount of dynamical friction the LMC has experienced and find that it is consistent with what is expected given the LMC's substantial dark matter halo. I will end with the future directions of this work and how these results can be used to constrain alternative gravity and alternative dark matter models.

Speaker: Denis Erkal (University of Surrey)
• 09:25
The stellar halo’s response as a probe of the LMC to MW mass ratio 15m

In the last two decades, some arguments have accumulated for a more important mass ratio of the Large Magellanic Cloud (LMC) to the Milky Way (MW) than was previously thought, of about 10% or more. This implies that the LMC has a measurable influence on the dynamics in the MW stellar halo, including both stellar densities and kinematics, as observed by Conroy et al. (2021) and Petersen et al. (2021). While this merger has been previously reproduced using N-body simulations (see, e.g., Garavito-Camargo et al., 2019), I will present the results of a recent study (Rozier et al., 2022) which aimed at modelling the merger via linear response theory. More specifically, we integrated the linearized collisionless Boltzmann-Poisson system of partial differential equations using a methodology known as the matrix method. Our results display the same large scale behaviour as state-of-the-art simulations, with a dipolar over/underdense pattern related to the reflex motion of the MW, as well as an overdense wake trailing behind the LMC. These results represent an efficient way of constraining the LMC to MW mass ratio, since this ratio is directly proportional (given the linear nature of the theory) to the amplitude of the relative density variations of the MW stellar halo, both in physical and in phase space. However, the amplitude of these variations may also depend on some model parameters, such as the structure of the MW potential (including a possible dark matter component), the initial density distribution of the stellar halo, as well as its initial internal kinematics. I will focus on the latter source of degeneracy, showing how the initial velocity anisotropy of the stellar halo impacts its response to the LMC. Interestingly enough, it appears that the physical space density of the (dipolar) reflex motion is insensitive to the stellar halo’s initial velocity anisotropy, and can therefore represent an efficient probe of the LMC to MW mass ratio.

Speaker: Simon Rozier (Observatoire Astronomique de Strasbourg)
• 09:40
The LMC-induced distortion of the Milky Way (and vice versa) 15m

The ongoing interaction of the Large Magellanic Cloud (LMC) and Milky Way (MW) allows for constraint of the mass (and profile) of both. Through comparison of models of the interaction and 6d halo star data, we determined that the LMC is currently both pulling the MW stellar disc away from the barycentre of the MW at 30 km/s, as well as inducing a measurable distortion in the outer halos of both the MW and LMC. These effects emphasise the need to move away from equilibrium models when attempting to measure the mass distributions of either. Models for the M33-M31 interaction -- a similar mass ratio -- reveal a markedly different interaction, with significantly smaller effects owing to the orbital history.

Speaker: Michael Petersen (University of Edinburgh)
• 09:55
The impact of massive satellites on the kinematic properties of the Local Group satellite galaxies 15m

The vast multidimensional data observed in the Local Group (LG) provide us with the unique opportunity of comparing the properties of the LG with their simulated analogs in cosmological simulations. In such a comparison it has been found that the observed kinematic properties of satellite galaxies in the LG are very unusual when compared to cosmological simulations. In both the Milky Way and the Andromeda galaxy, satellite galaxies are found to be distributed and moving in flattened co-rotating systems. A configuration that is only found in 0.3-3% of the simulated galaxies. Such disagreement is known as the planes of satellites problem. More intriguing is that the disagreement is persistent even in different dark matter models and unlike other 'small-scale problems' it is not sensitive to the inclusion of baryonic processes in the simulations.

In this talk, I would provide new evidence of how a natural explanation of the observed co-rotation patterns are associated with the out-of-equilibrium state of the galaxy. Recent missions and surveys, such as Gaia, have revealed that the Milky Way galaxy is not in dynamical equilibrium. The barycenter of the galaxy is moving due to the ongoing interaction with the Large Magellanic Cloud. Using the Latte hydrodynamical high-resolution zoom-in simulation of MW-like galaxies A massive satellite moves the reference frame of the inner halo with respect to the outer halo. Resulting in some cases in apparent co-rotation motions of the outer halo when observed from the disk of the galaxies. I'll show how the orbital poles distribution of outer halo objects is strongly affected during the pericenter passages. I will further discuss what properties of the mergers, mass ratios, pericenter passages, and eccentricities, would be needed to reproduce the observational co-rotations pattern observed in the Milky Way and M31.

These results provide new insight into how the observed co-ration patterns can inform us about the out-of-equilibrium stare of the galaxy. Which is a natural explanation that does not depend on the nature of the Dark Matter particle and in baryonic processes. In the talk, I will also discuss the importance of taking into account the dynamic state of the LG when comparing it with LG analogs in cosmological simulations.

Speaker: Nicolas Garavito Camargo (Flatiron Institute)
• 10:10
Multiband study of Shell region in the North-Eastern Small Magellanic Cloud using UVIT/AstroSat and Gaia 2m

The Small Magellanic Cloud (SMC) is one of the nearest, gas-rich interacting dwarf satellites of the Milky Way and the companion of the Large Magellanic Cloud (LMC). The interactions with the LMC and/or with the Milky Way play a significant role in the evolution of the SMC. With its widespread star formation and low metallicity, the SMC is one of the best test beds to study star formation and evolution in a tidally driven environment. The shell region located in the North-East outskirt of SMC is a tidally affected region where there has been recent star formation. Our aim is to understand the spatial distribution, age dating, and kinematics of the young population in this part of the tidally affected SMC disk.
We obtained far-UV (FUV) images of eleven fields in the North-East SMC Shell region using the UltraViolet Imaging Telescope (UVIT) on AstroSat. We created science-ready images and performed PSF photometry. We cross-matched the detected FUV stars with the Gaia EDR3 data and eliminated foreground stars to create an FUV catalog of a few thousand stars. We created FUV-optical color-magnitude diagrams and estimated the ages of the stellar population using isochrones to map the morphology, density, and tidal features of stars younger than ∼ 600 Myr. The identified episodes of star formation are used to constrain the details of the recent interaction of the SMC with the LMC. We also estimated the dispersion in the proper motion of the young and old stars to explore the kinematics of the North-East part of the outer SMC disk.

Speaker: Sipra Hota (Indian Institute of Astrophysics)
• 10:12
Direct Observations of the Primordial Magellanic Corona 2m

Recent measurements of a high mass for the LMC imply the LMC should host a massive Magellanic Corona, a collisionally ionized, warm-hot gaseous halo at the virial temperature $\sim10^{5.4}$ K initially extending out to the virial radius (100 - 130 kpc). Such a primordial Magellanic Corona would have shaped and fed the formation of the Magellanic Stream (e.g. Lucchini et al. 2020). Now, we have discovered direct observational evidence for this Magellanic Corona via highly ionized oxygen (O VI), and indirect detections via C IV and Si IV, seen in UV absorption toward background quasars using data from HST and FUSE (Krishnarao et al. 2022, Nature), We find the Magellanic Corona is part of a pervasive multiphase Magellanic CGM seen in many ionization states with a declining projected radial profile out to at least 35 kpc from the LMC and a total ionized CGM mass of $10^{(9.1 +/- 0.2)}M_\odot$. This independently confirms the large mass of the LMC as dynamically predicted. The evidence for the Magellanic Corona is a crucial step forward in characterizing the Magellanic Group and its nested evolution with the Local Group and will help us diagnose the impact of galactic scale winds emerging from star formation feedback in the LMC. In future work from an accepted HST Legacy Archival Program (PI: Kat Barger), we will directly measure and map this galactic scale outflow and the Magellanic Corona using hundreds of sightlines towards stars in the LMC from the ULYSSES program (Roman-Duval et al. 2020).

• 10:14
SMASHing the Small Magellanic Cloud: star formation history 2m

I present work on a spatially resolved, global star-formation history (SFH) of the Small Magellanic Cloud (SMC). I use the unprecedented deep photometric data (g~24 magnitude) from the Survey of the MAgellanic Stellar History (SMASH) survey utilising the Dark Energy Camera (DECAm) on the NOAO Blanco 4 m Telescope. The SFH is quantitatively obtained using colour-magnitude diagram (CMD) fitting techniques. For the first time, optical depth effects along the SMC's line-of-sight are considered in the SFHs derived.

I focus on the SFH of a shell-like structure located in the northeastern part of the SMC. I compare this SFH to the SFHs of several SMC regions. I discuss the results of such comparison with a focus on whether the recent and high star formation activity in the shell-like structure is correlated with the enhancements of star formation at young ages in the SMC. Finally, I put my results in the context of the tidal interactions between the SMC and its larger companion, the Large Magellanic Cloud (LMC) to shed light on the origins of the shell-like feature.

Speaker: Sakowska Joanna (University of Surrey)
• 10:16
Velocity dipoles in the halos of FIRE simulated galaxies 2m

One major source of disequilibrium in the Milky Way is its most massive satellite, the LMC. Kinematics of distant halo tracers show a velocity dipole in the Milky Way halo, which has been interpreted using N-body simulations as the LMC inducing a reflex motion in the Milky Way disk. In this talk, I discuss applying this framework to more realistic halos comprised of substructure from the FIRE-2 zoom-in cosmological simulations. Velocity dipoles are resolved in Milky Way-mass hosts experiencing an LMC-like interaction and evolve in a manner consistent with a two-body interaction between the stellar disk and the LMC analog. The magnitude of this dipole can be used to constrain the mass ratio of the Milky Way and LMC. However, satellite galaxies and stellar streams can create velocity dipoles in systems that aren't experiencing a major satellite accretion, suggesting that care must be taken to remove substructure in the Milky Way observations.

Speaker: Alex Riley (Durham University)
• 10:30 11:15
Poster Session & Coffee Break 45m
• 11:15 11:45
SESSION 4 : The Magellanic System: the Clouds, the Stream and the Leading Arm
Convener: Ekta Patel
• 11:15
The dynamical mass of the Large Magellanic Cloud as measured by its system of star clusters 15m

The Large and Small Magellanic Clouds (LMC and SMC) are the largest and most luminous dwarf satellite companions of the Milky Way. Due to their close proximity, they provide a unique opportunity to study the dynamics of their resolved stellar populations in unparalleled detail. Within the last years, high-precision proper motion measurements of stars within the Magellanic Clouds had a tremendous impact on our understanding of the Magellanic system and its relation to our own Galaxy. To date, however, the dynamics of the star cluster systems within the two dwarf galaxies has not received much attention.
In this contribution, I will introduce an ongoing observational campaign conducted by our team utilising the Hubble Space Telescope (HST) to precisely measure the proper motions of star clusters within the LMC. The exquisite resolution of HST allows us to measure precise proper motions of thousands of stars within each cluster.
The measured motions of the clusters, combined with photometric and spectroscopic measurements, will yield their full 6-dimensional phase space information within the LMC. I will present first results of the kinematic structures that are described by clusters of various ages and within different structural components of the galaxy. The motions of the clusters can further act as a tracer of the gravitational potential of the LMC and I will present preliminary measurements of the mass of the LMC resulting from the dynamics of the star clusters. Despite its importance in the field of Magellanic Cloud kinematics and evolution, the mass of the LMC is only vaguely known and several studies disagree on it. Our cluster-based study will provide an additional independent measurement of the LMC's mass.

Speaker: Florian Niederhofer (Leibniz-Institut für Astrophysik Potsdam (AIP))
• 11:30
Large-scale gas flows in the Local Group 15m

In this talk, I will discuss recent UV absorption-line measurements of large-scale gas flows in the Local Group with particular emphasis on the relation between LG galaxy kinematics and CGM/IGrM gas dynamics. New results from an HST/COS all-sky survey of gas in the Milky Way's CGM and the Local Group IGrM will be presented and compared with predictions from the HESTIA simulations .

Speaker: Philipp Richter (University of Potsdam)
• 11:45 12:45
Plenary Discussion 4 1h
Speakers: Maria-Rosa Cioni (Leibniz-Institüt für Astrophysik Potsdam), Roeland van der Marel (STScI)
• 12:45 13:30
Poster Session 45m
• 13:30 14:00
Lunch Break 30m
• 14:00 15:35
SESSION 5 : Near-field cosmology and galaxy masses beyond the Local Group
Convener: Angeles Pérez-Villegas
• 14:00
Stepping beyond our backyard: the era of Near-Field Cosmology 40m

The study of resolved stellar populations in the nearest galaxies, or "near-field cosmology", provides key constraints on the physics underlying galaxy formation and evolution. In this talk, I will present an overview of how deep, wide-field surveys of nearby groups of galaxies allow us to characterize the past and ongoing accretion processes shaping the halos of Milky Way-mass galaxies. This field is set to experience significant advancements with the current and future generations of state-of-the-art telescopes (JWST, Roman, TMT, VRO).

Speaker: Denija Crnojevic (The University of Tampa)
• 14:40
Dark matter content and dynamical masses of ultra-diffuse galaxies in the Coma cluster 15m

Ultra-diffuse galaxies (UDGs) are spatially extended, low surface brightness stellar systems with regular elliptical-like morphology found in a wide range of environments. Studies of the internal dynamics and dark matter content of UDGs that would elucidate their formation and evolution have been hampered by their low surface brightnesses. We identified a sample of low-mass early-type post-starburst galaxies in the Coma cluster still populated with young stars, which will passively evolve into UDGs in the next 5-10 Gyr. We collected deep observations for a large sample of low-mass galaxies in the Coma cluster using MMT Binospec. Here, we present spatially resolved velocity profiles out to 1 half-light radius, stellar velocity dispersions, ages, and metallicities for dozens of old UDGs in the Coma cluster and the same quantities derived out to 2-3 half-light radii for young dwarf post-starburst galaxies, the future UDGs. We derived their dynamical masses and dark matter content using Jeans modelling. High dark matter fractions, low degrees of rotational support, moderately low metallicities place UDGs onto the extension of the dwarf elliptical galaxy locus in several galaxy scaling relations such as the Fundamental Plane, the baryonic Tully-Fisher and the mass-metallicity relation. We demonstrate that statistically at least a half of present-day `old' UDGs were formed by ram-pressure stripping of disky progenitors. We discuss whether the same evolutionary scenario is applicable to the entire UDG population.

Speaker: Igor Chilingarian (Center for Astrophysics - Harvard and Smithsonian)
• 14:55
Decoding Dark Matter with Stellar Streams from Beyond the Milky Way 25m

In the coming decade, thousands of stellar streams will be observed in the halos of external galaxies with the Nancy Grace Roman Space Telescope, the Euclid Space Telescope, and the Vera C. Rubin Observatory. Stellar streams form when a dwarf galaxy or a cluster of stars is torn apart due to an underlying galactic potential, leaving behind a swath of thousands of stars that exhibit coherent, ordered motion. These streams are sensitive to the distribution of dark matter and to the population of dark matter subhalos in galaxies, both of which depend on the mass and interactions of the dark matter particle. In this talk, I discuss how to use the incoming wealth of stellar stream data to rule out dark matter candidates. I first focus on dwarf streams and present new models of the Centaurus A (Cen A) dwarf companion Dwarf 3 (Dw3) and Dw3's associated stellar stream. With a novel external galaxy stream-fitting technique, I show that there are many viable stream models that fit the data well, provided that Cen A has a dark matter halo mass larger than M_200 > 4.7 x 10^12 Msun. I also demonstrate that just one radial velocity measurement breaks degeneracies between stream morphology and dark matter halo mass. In the second part of the talk, I discuss stellar streams from globular clusters. Due to their low velocity dispersions, these streams are sensitive to gravitational interactions with low-mass dark matter subhalos. In the Milky Way, we know of a handful of stellar streams with noticeable under-densities, however, the Galactic bar, molecular clouds, and spiral arms can also lead to similar signatures in the streams. If we can instead find globular cluster streams in external galaxies without these baryonic perturbers, gaps in such streams can be easier to decipher and serve as a test of LCDM. I present the Hough Stream Spotter code which can rapidly and systematically search for linear structures in external galaxies. The Hough Stream Spotter combined with the Nancy Grace Roman Space Telescope will find hundreds of thin globular cluster streams in external galaxies. Lastly, I will discuss how to use stellar streams as tools to rule out dark matter candidates that are inconsistent with the new wealth of data.

Speaker: Sarah Pearson (NYU)
• 15:20
A story of galaxy mass assembly from dynamical modelling of nearby galaxies 2m

Understanding the mass assembly of galaxies is one of the big open questions in astronomy. A dynamical analysis of galaxies of the ATLAS3D survey provides new clues about the galaxy evolution process of galaxies as the sample comprises a good mix of fast and slow rotators with very different growing scenarios. Slow rotators are thought to accrete about 50 per cent of their stellar mass from satellite galaxies and their most massive progenitors have on average up to three major mergers during their evolution. Fast rotators in contrast, accrete less than 50 per cent and have on average less than one major merger in their past. But what is the imprint of the different evolutionary scenarios on the mass distribution and intrinsic shape of galaxies?

I will present a detailed dynamical study of the massive fast- and slow-rotator galaxies in the ATLAS3D survey with the Schwarzschild code DYNAMITE, that models galaxies as a superposition of their stellar orbits and colours those orbits with ages and metallicities. Using this full set of observables, I will quantify how tightly we can constrain the intrinsic shape and distribution of the visible and invisible matter in nearby early-type galaxies and relate it with the evolutionary scenarios. Compared to previous studies, triaxial modelling is essential for these galaxies to understand their complex kinematical features. I will conclude the discussion on what we can learn by comparison with dynamical studies of the Milky Way in order to learn about the dark past of galaxies.

Speaker: Sabine Thater (University of Vienna)
• 15:22
Dwarf Properties and Satellite Planes Beyond the Local Volume 2m

Dwarf galaxies are regarded as the oldest and most numerous galaxy type in the Universe, responsible for the formation of the higher mass galaxies we see today. While we know a lot about the properties of dwarfs in the Local Group as well as selected nearby groups and clusters, our understanding of these galaxies beyond the Local Volume is comparatively poor. The properties probed by this restricted range of locations may be statistically deviant and therefore investigating these objects in a large variety of density environments is critical towards a more complete understanding of galaxy formation and evolution. Through the study of dwarfs, a number of small-scale challenges of the $\Lambda$CDM paradigm have emerged over the years. While issues such as the missing satellite, the "Too-Big-to-Fail", and the cusp-core problem can increasingly be resolved by including baryonic physics and by altering the properties of dark matter, the so-called planes-of-satellites problem remains unsolved. Dwarf satellite galaxies in our Milky Way and different galaxy systems in the Local Volume appear to be arranged in thin, vast planes. It has been argued that these phase-space correlations can not be explained to a satisfactory degree by the standard model of cosmology but it is unclear whether these planes in our neighborhood are statistical outliers, or if they are perhaps a common phenomenon in the Universe. Recent deep imaging surveys have significantly increased the number of known dwarf galaxies and allow us to advance these tensions beyond the Local Volume. I will present our study analyzing the spatial distribution of 2210 dwarf galaxies identified in the MATLAS survey as well as results from follow-up observations with the MUSE instrument on the VLT. Spectral information for 56 of these dwarf galaxies, situated in low-to-medium density environments, allow for a deeper dive into their properties and for a comparison to the Local Volume dwarfs.

Speaker: Nick Heesters (EPFL)
• 15:24
Forming Blue Compact Dwarf Galaxy (BCD) through the Merger 2m

It has long been speculated that Blue Compact Dwarf galaxies (BCDs) are formed through the interaction between low-mass gas-rich galaxies, but due to a lack of evidence, this possibility has rarely been explored. We study a sample of compact star-forming dwarf galaxies that are selected from a merging dwarf galaxy catalog. We present a detailed study of their spectroscopic and structural properties. We find that these BCDs looking galaxies host extended stellar shells and thus is confirmed to be a dwarf-dwarf merger. Their stellar masses range between 8 × 107 Mʘ and 2 × 109 Mʘ. Although the extended tail and shell are prominent in the deep optical images, the overall major axis light profile is well modeled with a two-component Sersic function of inner compact and extended outer radii. We calculate the inner and outer component stellar-mass ratio using the two-component modeling. We find an average of 4:1 (with a range of 10:1 to 2:1) for our sample, indicating that these galaxies might have suffered a satellite accretion which triggers the starburst in the center of the host galaxies. From the measurement of Hα equivalent width, we derived the star-formation ages of these galaxies. The derived star-formation ages of these galaxies turn out to be less than 100 Myr, suggesting the recent ignition of star-formation due to events of satellite interaction.
Key words: galaxies: evolution-galaxies: irregular-galaxies: dwarf-galaxies: starburst-galaxies: interactions.

Speaker: Daya Nidhi Chhatkuli (Central Department of Physics, Tribhuvan University)
• 15:26
The north-south asymmetry of the ALFALFA HI velocity width function 2m

The number density of extragalactic 21-cm radio sources as a function of their spectral line-widths -- the HI width function (HI WF) -- is in principle a sensitive tracer of the dark matter halo mass function (HMF). The Λ cold dark matter model predicts that the HMF should be identical everywhere provided it is sampled in sufficiently large volumes, implying that the same should be true of the HI WF. The ALFALFA 21-cm survey measured the HI WF in two separate, northern (‘spring') and southern (‘fall') Galactic fields and found a systematically higher number density of sources in the spring field. Taken at face value, this is in tension with theoretical predictions. Using the Sibelius-DARK N-body simulation and the semi-analytical galaxy formation model GALFORM to create a mock ALFALFA survey, we find that the offset in number density likely has two origins: the sensitivity of the survey is different in the two survey fields, which has not been correctly accounted for in previous measurements; and the limited ability of the $1/V_\mathrm{eff}$ algorithm used for completeness corrections to mitigate biases arising from spatial clustering in the galaxy distribution. The latter bias is primarily driven by a foreground overdensity in the spring field within a distance of 30 Mpc, but more distant structure also plays a role. We provide an updated measurement of the ALFALFA HI WF (and HI MF) correcting for the variations in survey sensitivity. Only when systematic effects such as these are understood and corrected for can the HI WF fulfil its potential as a test of cosmological models.

Speaker: Richard Brooks (University College London)
• 15:28
Low-mass galaxy rotation curves that fail as dynamical mass tracers 2m

It is routinely assumed that galaxy rotation curves are equal to their circular velocity curves (modulo some corrections) such that they are good dynamical mass tracers. I will present the results of an unconventional, visualisation-driven analysis of 33 low-mass field galaxies from the APOSTLE suite of galaxy formation simulations exploring the limits of the validity of this assumption. Only 4/33 galaxies have HI rotation curves nearly equal to their circular velocity curves; the rest are undergoing a wide variety of dynamical perturbations of both secular and environmental origin. While some types of perturbations, such as ongoing mergers, have obvious observable signatures, others, such as wind from motion through the intergalactic medium, are much more subtle. Discrepancies between the rotation curves and circular velocity curves of low-mass galaxies have direct consequences for key results in near-field cosmology. They likely lead to an overestimation of the low-velocity end of the baryonic Tully-Fisher relation that is difficult to avoid (even by attempting to select 'equilibrium' galaxies), and could plausibly be the source of a significant portion of the observed diversity in low-mass galaxy rotation curve shapes.

Speaker: Kyle Oman (Durham University)
• 15:30
Dwarf early-type galaxies in the Coma and Virgo clusters: dynamical masses, kinematically-decoupled cores, stellar populations 2m

Galaxy clusters are the largest gravitationally bound structures in the Universe. Numerical simulations provide detailed scenarios on how they assemble and evolve over the lifetime of the Universe, but observational evidences supporting these predictions are still elusive. Galaxy populations in nearby clusters are dominated by dwarf stellar systems, and the number of these galaxies continues to grow over time even at the present epoch.
Over the last 4 years, using MMT Binospec we collected a rich spectroscopic dataset, which comprises over 250 dwarf early-type galaxies in three massive nearby clusters: Coma (D=99 Mpc), Abell 2147 (D=165 Mpc), and Abell 168 (D=193 Mpc). We have also reduced and analyzed spectra of dwarf galaxies in the Virgo cluster (D=16.5 Mpc) publicly available in the Keck, Gemini, and VLT data archives. For every galaxy we have a spatially resolved optical spectrum reaching 1-2 half-light radii from its center. By analyzing these data, we studied their internal properties, such as stellar kinematics (rotation, velocity dispersion), ages and chemical composition of their stars (e.g. to estimate when the star formation was quenched), and perform Jeans dynamical modelling, which yields dark matter content and dynamical masses. Profiles of radial velocity for a dozen of dEs in the Coma cluster demonstrate quite large kinematically decoupled cores suggestive of relatively recent mergers, which were experienced by these galaxies. We discuss several various scenarios of dE galaxy formation and evolution based on their dynamical masses, stellar population properties, internal dynamics and position within the host clusters and put them in correspondence with different dE sub-classes. With these data we can directly test the applicability of the abundance matching to galaxies in clusters in the 3e8-5e9 MSun range in stellar mass.

Speaker: Kirill Grishin (Astroparticule et Cosmologie)
• 15:35 16:00
Poster Session & Coffee Break 25m
• 16:00 16:30
Bus 1 to Caputh 30m

Leaves at gate to Telegrafenberg

• 16:30 18:00
Visit Einsteinhaus 1h 30m
• 16:45 17:15
Bus 2 to Caputh 30m

Leaves at gate to Telegrafenberg

• 18:30 22:30
Conference Dinner 4h

Kavalierhaus Caputh

• 22:30 22:50
Bus back to Potsdam 20m

Will take us back to Potsdam central station.

• Friday, 24 March
• 09:00 10:40
SESSION 5 : Near-field cosmology and galaxy masses beyond the Local Group
Convener: Laura Watkins
• 09:00
Mass models of nearby galaxies from cold gas dynamics 40m

In this talk I will review techniques and idiosyncrasies in building mass models of disk galaxies using cold gas dynamics. I will particularly focus on HI disks, which are generally more extended than stellar disks, so they allow tracing galaxy dynamics out to the most dark-matter-dominated regions. The combination of HI observations and near-infrared photometry, tracing the distribution of stellar mass, has proven extremely powerful to test both LCDM models of galaxy formation and modified gravity theories such as MOND.

Speaker: Federico Lelli (INAF)
• 09:40
On the baryon-driven contraction of dark matter halos 15m

The gravitational interaction between dark matter (DM) and baryons has long been ignored when building galaxies semi-empirically and observationally. In this talk, I will show that the baryonic gravity leads to an adiabatic contraction of DM halos, ignoring which would result in the built galaxies that are not in a dynamic equilibrium and hence cannot exist in reality. We propose a new approach to fitting galaxy rotation curves by numerically calculating the contraction of DM halos. We find adiabatic contraction makes DM halos more cuspy for massive galaxies, so that their rotation curves cannot be fit without systematically reducing baryonic contributions. We also examine the baryonic effect on the predicted radial acceleration relation of cold DM model, and find the predicted relations of massive galaxies are systematically higher than observed. Both tensions point to a core-cusp problem, a classical problem for dwarf galaxies but persisting in massive galaxies as well due to strong adiabatic contraction. In order to reconcile this problem, feedback must work in massive galaxies as efficiently as in dwarf galaxies.

Speaker: Pengfei Li (Leibniz-Institute for Astrophysics at Potsdam)
• 09:55
The oblateness of the dark matter halos of nearby galaxies and its correlation with gas mass fractions 15m

We present a method to measure the the oblateness parameter q of the dark matter halo of gas rich galaxies that have extended HI disks. We have applied our model to a sample of 20 nearby galaxies that are gas rich and close to face-on, of which 6 are large disk galaxies, 8 have moderate stellar masses and 6 are low surface brightness dwarf galaxies. We have used the stacked HI velocity dispersion and HI surface densities to derive q in the outer disk regions. Our most important result is that gas dominated galaxies (such as LSB dwarfs) that have M(gas)/M(baryons)>0.5 have oblate halos (q < 0.55), whereas stellar dominated galaxies have a range of q values from 0.2 to 1.3. We also find a significant positive correlation between q and stellar mass, which indicates that galaxies with massive stellar disks have a higher probability of having halos that are spherical or slightly prolate, whereas low mass galaxies preferably have oblate halos.

Speaker: Mousumi Das (Indian Institute of Astrophysics)
• 10:10
EDGE: Predicting Scatter in the Stellar Mass–Halo Mass Relation for Dwarf Galaxies 15m

One of the primary hurdles in pushing dark matter constraints to dwarf scales the uncertainty in the stellar-mass--halo-mass (SMHM) relation. Results from simulations differ by two orders of magnitude at halo masses < 10^10 solar masses, and none can match observations. Moreover, there is no consensus on the amount of scatter. To address these uncertainties, I used high-resolution simulations of dwarfs from the Engineering Dwarfs at Galaxy formation's Edge (EDGE) Project to investigate the SMHM at < 10^10 solar masses. Based on these simulations, I created a new galaxy-halo model, DarkLight, that can accurately predict the stellar masses of dwarfs, uniquely including its dependence on accretion history. We find that a ~1 dex scatter in the SMHM relation for halos with a mass of 10^9 solar masses is due predominantly to the scatter in the accretion histories. Differences between previous simulations results and their inability to reproduce observations can be explained by an increase in scatter in the SMHM at low masses, the limited sample sizes and resolution of simulations, and the incompleteness of observational searches. We discuss the implications this has for constraining dark matter models with assembly history statistics that differ from cold dark matter, such as warm dark matter.

Speaker: Stacy Kim (University of Surrey)
• 10:25
Too Big to Fail? 6D Stellar Streams in the Milky Way and Cosmological Simulations 15m

Recent large photometric, astrometric, and spectroscopic surveys have enabled the first systematic observations of Milky Way stellar streams in 6D. At the same time, cutting edge cosmological simulations are now at resolutions that allow for the study of dwarf galaxy streams around Milky Way-like hosts. In this talk, I will present the discovery and characterization of a population of 6D stellar streams with observations from the Dark Energy Survey, Gaia, and the Southern Stellar Stream Spectroscopic Survey (S5) in comparison to streams identified in cosmological simulations. These comparisons enable deeper studies of satellite populations and tidal disruption in simulations and observations, and further tests of the small-scale challenges to LCDM.

Speaker: Nora Shipp (Massachusetts Institute of Technology)
• 10:40 11:20
Poster Session & Coffee Break 40m
• 11:20 13:00
SESSION 5 : Near-field cosmology and galaxy masses beyond the Local Group
Convener: Laura Watkins
• 11:20
The coherent motion of Cen A dwarf satellite galaxies as a challenge for cosmology 25m

The plane-of-satellites problem is one of the most severe small-scale challenges for the standard Λ cold dark matter (ΛCDM) cosmological model: Several dwarf galaxies around the Milky Way and Andromeda co-orbit in thin, planar structures. A similar case has been identified around the nearby elliptical galaxy Centaurus A (Cen A). We studied the satellite system of Cen A with line-of-sight velocities from VLT/MUSE observations and TRGB distances from VLT/FORS2 and HST observations. Out of 28 dwarf galaxies with measured velocities 21 share a coherent motion and are arranged in a flattened structure. Similarly, flattened and coherently moving structures are found only in 0.2% of Cen A analogs in the Illustris-TNG100 cosmological simulation, independently of whether we use its dark-matter-only or hydrodynamical run. These analogs are not co-orbiting, and they arise only by chance projection, thus they are short-lived structures in such simulations. Our findings indicate that the observed co-rotating planes of satellites are a persistent challenge for ΛCDM, which is largely independent from baryon physics.

Speaker: Oliver Müller (Institute of Physics, Laboratory of Astrophysics, École Polytechnique Fédérale de Lausanne (EPFL))
• 11:45
Centaurus A, its globular clusters, planetary nebulae and dark matter halo shape 15m

The mass content of the Universe is dominated by non-baryonic dark matter, according to the Lambda Cold Dark Matter cosmology interpretation of observational evidence. However, not all observations agree with the theory and many predictions remain difficult to investigate. In particular, simulations predict that the shapes of the most massive dark matter haloes deviate from spherical symmetry. The outer regions of galaxies show the most prominent signatures of dark matter. Discrete dynamical modeling of halo tracers offers a unique opportunity to further investigate the presence and large-scale distribution of dark matter in galactic haloes. Extended halo populations of globular clusters (GCs) and planetary nebulae (PNe) are ideal kinematic tracers in the outer regions of the galaxies and can be observed out to 10 or even 15 effective radii in several galaxies. Using both tracers that have distinct kinematic properties and spatial distribution helps in breaking the well-known mass-anisotropy degeneracy that hinders dynamical modeling. In this work we constrain, the flattening of the dark matter halo of Centaurs A galaxy (NGC5128). In my talk I will present the analysis of the photometric and kinematic properties of PNe and GCs and the results from the discrete anisotropic Jeans modeling.

Speaker: Tadeja Veršič (ESO, University of Vienna)
• 12:00
New Satellite Dwarf Galaxies of NGC2683 and M104 and the Search for Satellite Planes. 15m

Since the early 2000s satellite dwarf galaxies of the Local Group and more recently, the nearby Centaurus A/M83 group, have been known to show morphological characteristics and spatial distributions that do not match predictions from ΛCDM simulations. In particular, satellite dwarf galaxies inhabiting the Local Group and the Centaurus A/M83 group appear to be co-rotating in confined disks known as satellite planes. ΛCDM simulations have failed to fully reproduce satellite planes despite increasingly advanced simulations, including baryon physics, and aren’t associated with galactic analogues of the local universe in ΛCDM simulations. Several presented hypotheses suggest that these apparent discrepancies may be the result of one of three mechanisms; that statistical bias in determinations of satellite planes in the local universe leads to inflated significance, that simulations fail to simulate some feature of the universe or that unique conditions in the local universe led to the formation of satellite planes. This is motivating an international effort to search for satellite galaxies and ultimately, satellite planes, outside of the local universe. Using the Hyper Suprime Cam from the Subaru telescope, we capture ~4 degree field of view images of nearby isolated L* galaxy environments with the intent of identifying satellite dwarf galaxies complete to an absolute g-band magnitude of >-10 and ultimately, identifying satellite planes. These environments reside outside of the Local Sheet, which may have provided the conditions that favours the generation of satellite planes, or the galaxies within may not form an independent sample which reduces the significance of satellite planes. Our recent research has focused on searching for the satellite galaxies of M104 and NGC2683, two mostly isolated galaxy environments that reside outside of the local sheet and are thus free from biases associated with it. Across these two environments, we find over 20 newly discovered highly probably dwarf galaxy candidates up to projected radii of 400 kpc, which for NGC2683 are distributed in an anisoptric, flattened disk and for M104, a lop-sided but circular disk. We intend to follow up these candidates with observations using IFU-M, a novel IFU spectroscope undergoing commissioning at the Magellan telescopes of the Las Campanas Observatory in Chile. These spectroscopic observations will confirm that our new galaxy candidates are associated with their assumed hosts, determine if co-rotation is present, and for brighter candidates, allow us to explore the mass-to-light ratios and star formation histories of these dwarf galaxies. This spectroscopically enhanced dataset of newly discovered dwarf satellite galaxies limits statistical bias and enables us to answer questions about the local universe and our cosmological models; is the local universe a cosmological oddity, or do our models of the universe fail to account for an unknown factor?

Speaker: Ethan Crosby (ANU)
• 12:15
Precise constrained cosmological simulations of the Local Group 15m

Cosmological simulations have been used to understand the formation of structure in the LCDM paradigm on small and large scales. Most simulations start with unconstrained Gaussian initial conditions, and therefore generically do not produce good analogues of the Local Group at present day. While constrained simulations exist, these have difficulty in precisely satisfying all our observational constraints on the Local Group, and their result is not an unbiased and fair sample of the posterior distribution of LCDM universes subject to the observational constraints of the Local Group. Some applications of such a sample include putting into cosmological context our distribution of satellites, the alignment of the dark-matter haloes and their spin and the relation to the assembly history, and to determine the effect of our environment on the Local Group's spatial configuration and kinematics.

In this work, we extend the BORG algorithm (Bayesian Origin Reconstruction from Galaxies), that has already been used to model the Local Large-Scale Structure, to reconstruct the Local Group. Using this toolset, we perform a statistical inference on the history of the Local Group, following a LCDM prior on the cosmological initial conditions, and a likelihood that constrains local observational quantities, like the masses, positions, and velocities of the Milky Way and Andromeda haloes. In the near future, we plan to embed our Local Group in a realistic large-scale structure as well. To the best of our knowledge, this is the first time a cosmological simulation has been able to reproduce all these properties simultaneously with high precision.

Speaker: Ewoud Wempe (Kapteyn Astronomical Institute)
• 12:30
Forming the Local Group satellite planes in a past Milky Way-Andromeda flyby 15m

The satellite galaxy systems of the Milky Way (MW) and M31 both show a very thin kinematically coherent structure or satellite plane. It has previously been shown that each plane is in 3.55σ tension with ΛCDM expectations, which combined with the similar structure around Centaurus A falsifies the model at 5.3σ confidence. In this talk, I will present hydrodynamical simulations of the MW and M31 in Milgromian dynamics (MOND), which requires them to have experienced a past close flyby roughly 9 Gyr ago. While the formation of tidal dwarf galaxies is not resolved, the tidal debris around each galaxy ends up distributed anisotropically. In each case, the preferred orientation matches that of the actually observed satellite galaxy plane (MNRAS, 513, 129). The MW and M31 retain thin discs with realistic scale lengths, orientations, and present separation. I will therefore argue that the Local Group satellite planes are best understood as arising from a past close MW-M31 flyby in MOND, whose enhancement to gravity at low accelerations naturally explains the apparent dark matter content of the satellite plane members. This contrasts with the Newtonian picture where tidal dwarfs should lack dark matter, while primordial dwarfs should be distributed almost isotropically and should moreover constitute the dominant satellite population.

Speaker: Indranil Banik (University of Saint Andrews)
• 12:45
A dynamical mass map of the nearby Fornax galaxy cluster 15m

The Fornax cluster provides an unparalleled opportunity to investigate the formation and evolution of early-type galaxies in a dense environment. Using the spectroscopic data from the Visible Multi-Object Spectrograph at Very Large Telescope (VLT/VIMOS) from the FVSS survey, we have kinematically characterised the photometrically detected globular cluster (GC) candidates in the core of the cluster. We confirm a total of 777 GCs new velocity measurements. Combined with previous literature, radial velocity measurements of GCs in Fornax, we compile the most extensive spectroscopic GC sample of 2341 objects in this environment.

With the final goal of understanding the mass assembly of the Fornax galaxy cluster, we are using our GC radial velocity catalogue to perform dynamical mass modelling of NGC1399 out to 200 kpc (∼ 6 𝑟𝑒𝑓𝑓 of NGC1399). Using the spherical Jeans modelling, we have performed the dispersion-kurtosis modelling to obtain the mass profile of NGC1399 and the orbital anisotropy of GCs. We have investigated the effect of the intra-cluster GCs in the mass-modelling results.

We find that both cusp (NFW) and core (Burkert) dark matter (DM) halo can produce the observed kinematics. Including the intra-cluster GCs in mass-modelling analysis produces a heavier DM halo. Independent of the DM halo profiles used in modelling, we find that GCs in intra-clusters have mild radial anisotropy, especially for the blue GCs. In this talk, I will discuss the baryonic and dark matter distribution of the Fornax galaxy cluster out to half of its virial radius. Specifically, I will talk about the impact of the intra-cluster GCs on the Fornax cluster mass profile. In addition, I will present the orbital distribution of the intra-cluster GCs in the Fornax assembly, showing their accreted nature.

Speaker: Avinash Chaturvedi (European Southern Observatory)
• 13:00 13:30
Poster Prize Presentations 30m
• 13:30 14:30
Lunch Break 1h
• 14:30 15:30
Plenary Discussion 5 1h
Speakers: Marina Rejkuba (ESO), Stacy McGaugh (Case Western Reserve University)
• 15:30 16:00
Closing Remarks 30m
Speakers: Francois Hammer, Marcel Pawlowski (AIP)