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 3π imaging survey. The systemic velocities of the three systems (v r, helio = -198.4 ± 1.1 km s-1, -371.6 ± 0.7 km s-1, and -326 ± 3 km s-1, 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 (σvr = 10.3 ± 0.9 km s-1 and 8.4 ± 0.6 km s-1, respectively), leading to dynamical-mass estimates for both of ~4 × 107 M ⊙ 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 three 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.