The splash survey: A spectroscopic analysis of the metal-poor, low-luminosity M31 dSph satellite andromeda X

Andromeda X (And X) is a newly discovered low-luminosity M31 dwarf spheroidal galaxy (dSph) found by Zucker etal. in the Sloan Digital Sky Survey (SDSS; York etal.). In this paper, we present the first spectroscopic study of individual red giant branch stars in And X, as a part of the Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo (SPLASH) Survey. Using the Keck II telescope and multiobject DEIMOS spectrograph, we target two spectroscopic masks over the face of the galaxy and measure radial velocities for 100 stars with a median accuracy of σ_v 3 km s^-1. The velocity histogram for this field confirms three populations of stars along the sight line: Foreground Milky Way dwarfs at small negative velocities, M31 halo red giants over a broad range of velocities, and a very cold velocity "spike" consisting of 22 stars belonging to And X with v_rad = -163.8±1.2 km s^-1. By carefully considering both the random and systematic velocity errors of these stars (e.g., through duplicate star measurements), we derive an intrinsic velocity dispersion of just σ_v = 3.9±1.2 km s^-1 for And X, which for its size, implies a minimum mass-to-light ratio of M/L_V = 37 ⁺²⁶ _-19 assuming that the mass traces the light. Based on the clean sample of member stars, we measure the median metallicity of And X to be [Fe/H] = -1.93±0.11, with a slight radial metallicity gradient. The dispersion in metallicity is large, σ([Fe/H]_phot) = 0.48, possibly hinting that the galaxy retained much of its chemical enrichment products. And X has a total integrated luminosity (M_V = -8.1±0.5) that straddles the classical Local Group dSphs and the new SDSS ultra-low luminosity galaxies. The galaxy is among the most metal-poor dSphs known, especially relative to those with M_V < -8, and has the second lowest intrinsic velocity dispersion of the entire sample. Our results suggest that And X is less massive by a factor of 4 when compared to Milky Way dSphs of comparable luminosity (e.g., Draco and Ursa Minor). We discuss the potential for better understanding the formation and evolution mechanisms for M31's system of dSphs through (current) kinematic and chemical abundance studies, especially in relation to the Milky Way sample.