THE DISTRIBUTION OF ALPHA ELEMENTS IN ANDROMEDA DWARF GALAXIES

Abstract

We present alpha to iron abundance ratios for 226 individual red giant branch stars in nine dwarf galaxies of the Andromeda (M 31) satellite system. The abundances are measured from the combined signal of Mg, Si, Ca, and Ti lines in Keck/DEIMOS medium-resolution spectra. This constitutes the first large sample of alpha abundance ratios measured in the M 31 satellite system. The dwarf galaxies in our sample exhibit a variety of alpha abundance ratios, with the average values in each galaxy ranging from approximately solar ([alpha/Fe] ~ +0.0) to alpha-enhanced ([alpha/Fe] ~ +0.5). These variations do not show a correlation with internal kinematics, environment, or stellar density. We confirm radial gradients in the iron abundance of two galaxies out of the five with sufficient data (NGC 185 and And II). There is only tentative evidence for an alpha abundance radial gradient in NGC 185. We homogeneously compare our results to the Milky Way classical dwarf spheroidals, finding evidence for wider variation in average alpha abundance. In the absence of chemical abundances for the M 31 stellar halo, we compare to the Milky Way stellar halo. A stellar halo comprised of disrupted M31 satellites is too metal-rich and inconsistent with the Milky Way halo alpha abundance distribution even if considering only satellites with predominantly old stellar populations. The M 31 satellite population provides a second system in which to study chemical abundances of dwarf galaxies and reveals a wider variety of abundance patterns than the Milky Way.