Andromeda and the Seven Dwarfs: M31 Mass, Satellite Orbits, and the Nature of the Satellite Plane

Proper motions (PMs) from HST and Gaia have revolutionized the field of Galactic archaeology in the Milky Way (MW), but PM studies in the M31 system are still in their early stages. Gaia can only detect the brightest stars in star forming regions at the distance of M31, so HST is the only observatory currently capable of measuring PMs of M31 satellite dwarf spheroidals. We propose to obtain second epoch imaging of 7 M31 satellites (And I, II, III, VII, XV, XVI, XXVIII), targeting the same fields with earlier deep HST observations to measure PMs with uncertainties of 40-50 km/s. We will use the PM-based 6D phase space information of satellites to constrain the total M31 mass with uncertainties at the 30% level. We will also test the dynamical stability of the Great Plane of Andromeda using our PM results for And I, III, and XVI. This will provide clues to the origin of satellite planes observed around M31 as well as the MW. The orbits derived for individual satellites will allow us to disentangle the processes that have contributed to the quenching of star formation, connect our target galaxies to M31 tidal structures, and test the merger scenario of And II. Altogether, our proposed measurements will nearly triple the sample of M31 satellites with available PMs, dramatically improving our understanding of the M31 halo and its satellite system. A revised estimate for the mass of M31 will also improve our understanding of the dynamics in the Local Group (LG), and set an important benchmark for studies of satellite systems beyond the LG. Finally, the results from this program will inform us how representative the MW is in a cosmological context.