The SPLASH Survey: A Spectroscopic Analysis of the Metal-Poor, Low-Luminosity M31 dSph Satellite Andromeda X

TL;DR

This study presents the first spectroscopic analysis of Andromeda X, revealing its velocity, mass-to-light ratio, and metallicity, providing insights into its structure and chemical evolution.

Contribution

It offers the first detailed spectroscopic measurements of individual stars in Andromeda X, including velocity dispersion and metallicity, advancing understanding of low-luminosity M31 dwarf spheroidals.

Findings

Velocity dispersion of 3.9 km/s for And X

Mass-to-light ratio estimated at 37+26-19

Median metallicity of [Fe/H] = -1.93

Abstract

Andromeda X (And X) is a newly discovered low-luminosity M31 dwarf spheroidal galaxy (dSph) found by Zucker et al. (2007) in the Sloan Digital Sky Survey (SDSS - York et al. 2000). In this paper, we present the first spectroscopic study of individual red giant branch stars in And X, as a part of the SPLASH Survey (Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo). 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 sigma_v ~ 3 km/s. 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. 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 sigma_v = 3.9 +/- 1.2 km/s for And X, which for its size, implies a minimum mass-to-light ratio of M/L =37^{+26}_{-19} assuming 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, sigma([Fe/H]) = 0.48, possibly hinting that the galaxy retained much of its chemical enrichment products. 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. (abridged version)