Multi-Element Abundance Measurements from Medium-Resolution Spectra. IV. Alpha Element Distributions in Milky Way Dwarf Satellite Galaxies

TL;DR

This study analyzes alpha element abundance patterns in eight Milky Way dwarf spheroidal galaxies to understand their star formation histories and chemical evolution, revealing that Type Ia supernovae influence most stars and linking galaxy properties to their evolution.

Contribution

It provides new insights into the chemical evolution of dwarf spheroidal galaxies by analyzing a large stellar sample and developing a comprehensive chemical evolution model.

Findings

Type Ia supernovae contribute to most stars' abundances, even at low metallicities.

No observed 'knees' in [alpha/Fe] vs. [Fe/H] diagrams, contrary to predictions.

Gas loss and supernova feedback shape the evolution, correlating with galaxy luminosity.

Abstract

We derive the star formation histories of eight dwarf spheroidal (dSph) Milky Way satellite galaxies from their alpha element abundance patterns. Nearly 3000 stars from our previously published catalog (Paper II) comprise our data set. The average [alpha/Fe] ratios for all dSphs follow roughly the same path with increasing [Fe/H]. We do not observe the predicted knees in the [alpha/Fe] vs. [Fe/H] diagram, corresponding to the metallicity at which Type Ia supernovae begin to explode. Instead, we find that Type Ia supernova ejecta contribute to the abundances of all but the most metal-poor ([Fe/H] < -2.5) stars. We have also developed a chemical evolution model that tracks the star formation rate, Types II and Ia supernova explosions, and supernova feedback. Without metal enhancement in the supernova blowout, massive amounts of gas loss define the history of all dSphs except Fornax, the most luminous in our sample. All six of the best-fit model parameters correlate with dSph luminosity but not with velocity dispersion, half-light radius, or Galactocentric distance.