The Distribution of Alpha Elements in Ultra-Faint Dwarf Galaxies

TL;DR

This study measures alpha-element abundances in 61 stars across 8 ultra-faint dwarf galaxies, revealing their chemical enrichment history and differences from the Milky Way halo.

Contribution

It provides the largest dataset of alpha-element abundances in ultra-faint dwarf galaxies, including first measurements for several galaxies, and analyzes their chemical evolution.

Findings

Most UFDs show decreasing [alpha/Fe] with increasing [Fe/H], indicating Type Ia supernova enrichment.

Even the faintest UFDs experienced some chemical self-enrichment.

UFDs' alpha abundance patterns differ from the Galactic halo and more luminous dwarf galaxies.

Abstract

The Milky Way ultra-faint dwarf galaxies (UFDs) contain some of the oldest, most metal-poor stars in the Universe. We present [Mg/Fe], [Si/Fe], [Ca/Fe], [Ti/Fe], and mean [alpha/Fe], abundance ratios for 61 individual red giant branch stars across 8 UFDs. This is the largest sample of alpha abundances published to date in galaxies with absolute magnitudes M_V > -8, including the first measurements for Segue 1, Canes Venatici II, Ursa Major I, and Leo T. Abundances were determined via medium-resolution Keck/DEIMOS spectroscopy and spectral synthesis. The sample spans the metallicity range -3.4 < [Fe/H] < -1.1. With the possible exception of Segue 1 and Ursa Major II, the individual UFDs show on average lower [alpha/Fe] at higher metallicities, consistent with enrichment from Type Ia supernovae. Thus even the faintest galaxies have undergone at least a limited level of chemical self-enrichment. Together with recent photometric studies, this suggests that star formation in the UFDs was not a single burst, but instead lasted at least as much as the minimum time delay of the onset of Type Ia supernovae (~100 Myr) and less than ~2 Gyr. We further show that the combined population of UFDs has an [alpha/Fe] abundance pattern that is inconsistent with a flat, Galactic halo-like alpha abundance trend, and is also qualitatively different from that of the more luminous CVn I dSph, which does show a hint of a plateau at very low [Fe/H].