A 4D View of the Local Group

The dwarf galaxies around the Milky Way are extremely valuable laboratories for studying the nature of dark matter, the threshold for galaxy formation, and chemical evolution in the early universe.  After reviewing the revolution in our understanding of the Milky Way's satellite population that has resulted from recent wide-field imaging surveys, I will discuss new proper motion measurements of nearby dwarfs using the Gaia satellite.  Gaia has made it possible to instantly determine the proper motions of galaxies out to distances beyond 100 kpc.  The velocities for dwarfs within this distance are generally determined to better than 50 km/s, even when only a handful of known member stars are bright enough for Gaia measurements.  I use the resulting 3D space motions for each dwarf to compute their orbits around the Milky Way and extrapolate backward in time.  The orbital velocities are larger than expected, suggesting that the mass of the Milky Way may be on the high end of the previously estimated range. The orbits of several dwarf galaxies clearly reveal that they are satellites of the Magellanic Clouds, opening up the possibility of studying the effect of environment on evolution of the faintest dwarfs.  I examine the effect of tidal forces on each dwarf and show that only a small fraction of the Milky Way satellite population can have suffered significant tidal stripping.