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 _ { \mathrm { r,helio } } = -198.4 \pm 1.1 { km s ^ { -1 } } , -371.6 \pm 0.7 { km s ^ { -1 } } , and -326 \pm 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 ( \sigma _ { \mathrm { vr } } = 10.3 \pm 0.9 { km s ^ { -1 } } and 8.4 \pm 0.6 { km s ^ { -1 } } , respectively ) , leading to dynamical-mass estimates for both of \sim 4 \times 10 ^ { 7 } { M _ { \odot } } 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 \pm 0.1 , -1.7 \pm 0.1 , and -2.0 \pm 0.2 , respectively ) .
All these properties are typical of dwarf galaxy satellites of Andromeda with their luminosity and size .