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 ( \sim 100 Myr ) and less than \sim { 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 ] .