Spectroscopy of the three distant Andromedan satellites Cassiopeia III,   Lacerta I, and Perseus I
  We present Keck II/DEIMOS spectroscopy of the three distant dwarf galaxies of M31 Lacerta I, Cassiopeia III, and Perseus I, recently discovered within the Pan-STARRS1 3\pi imaging survey. The systemic velocities of the three systems (v_{r,helio} = -198.4 +/- 1.1 km/s, -371.6 +/- 0.7 km/s, and -326 +/- 3 km/s, respectively) confirm that they are satellites of M31. In the case of Lacerta I and Cassiopeia III, the high quality of the data obtained for 126 and 212 member stars, respectively, yields reliable constraints on their global velocity dispersions (\sigma_{vr} = 10.3 +/- 0.9 km/s and 8.4 +/- 0.6 km/s, respectively), leading to dynamical-mass estimates for both of ~4x10^7 Msun within their half-light radius. These translate to V-band mass-to-light ratios of 15^{+12}_{-9} and 8^{+9}_{-5} in solar units. We also use our spectroscopic data to determine the average metallicity of the 3 dwarf galaxies ([Fe/H] = -2.0 +/- 0.1, -1.7 +/- 0.1, and -2.0 +/- 0.2, respectively). All these properties are typical of dwarf galaxy satellites of Andromeda with their luminosity and size. 
Nicolas F. Martin, Kenneth C. Chambers, Michelle L. M. Collins,   Rodrigo A. Ibata, R. Michael Rich, Eric F. Bell, Edouard J...
astro-ph.GA