IAUS379: Dynamical Masses of Local Group Galaxies

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.