Spectroscopy of the three distant Andromedan satellites Cassiopeia III, Lacerta I, and Perseus I

TL;DR

This study uses Keck II/DEIMOS spectroscopy to analyze three distant Andromedan satellite galaxies, confirming their association with M31 and providing detailed measurements of their velocities, masses, and metallicities.

Contribution

First detailed spectroscopic analysis of Lacerta I, Cassiopeia III, and Perseus I, providing their velocities, masses, and metallicities, confirming their status as M31 satellites.

Findings

Confirmed all three as satellites of M31 via systemic velocities.

Derived velocity dispersions and dynamical masses for Lacerta I and Cassiopeia III.

Measured metallicities consistent with typical dwarf galaxy satellites.

Abstract

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.