The Distribution of Alpha Elements in Ultra-Faint Dwarf Galaxies
  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].
