The Keck/DEIMOS Stellar Archive: II. Dynamical Masses and Metallicities for a Uniform Sample of Milky Way Satellites

TL;DR

This paper presents a large, homogeneous dataset of spectroscopic measurements for Milky Way satellites, enabling more accurate studies of their dark matter content, metallicities, and galaxy formation thresholds.

Contribution

It provides the largest self-consistent sample of dynamical masses and metallicities for Milky Way satellites, derived from a uniform re-analysis of Keck/DEIMOS data, improving previous heterogeneous analyses.

Findings

Satellite galaxies are distinct from globular clusters in mass-to-light ratios.

Average densities of satellites match CDM model predictions.

A break in the mass-metallicity relation occurs near log M*/Msun = 4.

Abstract

Population-level studies of Milky Way satellites used to constrain dark matter or the threshold of galaxy formation often rely on velocity dispersions and metallicities derived from heterogeneous spectroscopic analyses. Systematic differences between data reduction pipelines and membership criteria can masquerade as astrophysical signals, or obscure real trends. Here, we present the largest self-consistent sample of spectroscopically-derived quantities for Milky Way satellite galaxies and globular clusters based on a homogeneous re-analysis of individual stars observed with the Keck/DEIMOS spectrograph. We determine enclosed dynamical masses, mean [Fe/H] metallicities, and metallicity dispersions for 67 systems with 10 or more member stars. At a given stellar mass, systems classified as satellite galaxies are well separated from globular clusters in their dynamical mass and mass-to-light ratios. The average enclosed mass densities of satellite galaxies agree with semi-analytic CDM model predictions. For satellite galaxies, we observe a break in the stellar mass-metallicity relation near log M*/Msun = 4 (M_V ~ -4.5). Above this stellar mass, satellite galaxies show the well-known tight trend (0.16 dex scatter in [Fe/H]) of decreasing metallicity with stellar mass; below log M*/Msun = 4, the mass-metallicity relation flattens and/or increases in scatter. Satellite galaxies have internal metallicity scatter between 0.3-0.4 dex across our stellar mass range. These uniform measurements will enable tighter constraints on the stellar mass-halo mass relation, improved J-factor estimates for dark matter searches, and lay a foundation for interpreting the flood of new Milky Way satellites expected in the LSST/Roman/Euclid era.