IAUS379: Dynamical Masses of Local Group Galaxies

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.