IAUS379: Dynamical Masses of Local Group Galaxies

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.