2014ApJ...793L..14M

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, -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 (σ_vr= 10.3±0.9 km/s and 8.4±0.6 km/s, respectively), leading to dynamical-mass estimates for both of ∼4x10⁷ M_☉ within their half-light radius. These translate to V-band mass-to-light ratios of 15_–9⁺¹² and 8_–5⁺⁹ 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.