The Outer Limits of the M31 System: Kinematics of the Dwarf Galaxy Satellites And XXVIII and And XXIX

We present Keck/DEIMOS spectroscopy of resolved stars in the M31 satellites And XXVIII & And XXIX. We show that these are likely self-bound galaxies based on 18 and 24 members in And XXVIII & And XXIX, respectively. And XXVIII has a systemic velocity of -331.1 +/- 1.8 km/s and velocity dispersion of 4.9+/-1.6 km/s, implying a mass-to-light ratio (within r_1/2) of ~ 44 +/- 41. And XXIX has a systemic velocity of -194.4 +/- 1.5 km/s and velocity dispersion of 5.7+/-1.2 km/s, implying a mass-to-light ratio (within r_1/2) of ~ 124 +/- 72. The internal kinematics and implied masses of And XXVIII & And XXIX are similar to dwarf spheroidals (dSphs) of comparable luminosities, implying that these objects are dark matter-dominated dwarf galaxies. Despite the large projected distances from their host (380 and 188 kpc), the kinematics of these dSph suggest that they are bound M31 satellites.