High-Resolution Spectroscopy of Extremely Metal-Poor Stars in the Least
  Evolved Galaxies: Ursa Major II and Coma Berenices
  We present Keck/HIRES observations of six metal-poor stars in two of the
ultra-faint dwarf galaxies orbiting the Milky Way, Ursa Major II and Coma
Berenices. These observations include the first high-resolution spectroscopic
observations of extremely metal-poor stars ([Fe/H]<-3.0) stars not belonging to
the Milky Way (MW) halo field star population. We obtain abundance measurements
and upper limits for 26 elements between carbon and europium. The entire sample
of stars spans a range of -3.2<[Fe/H]<-2.3, and we confirm that each galaxy
contains a large intrinsic spread of Fe abundances. A comparison with MW halo
stars of similar metallicities reveals substantial agreement between the
abundance patterns of the ultra-faint dwarf galaxies and the MW halo for the
light, alpha and iron-peak elements (C to Zn). This agreement contrasts with
the results of earlier studies of more metal-rich stars (-2.5<[Fe/H]<-1.0) in
more luminous dwarf spheroidal galaxies (dSphs), which found significant
abundance discrepancies with respect to the MW halo data. The abundances of
neutron-capture elements (Sr to Eu) in the ultra-faint dwarf galaxies are
extremely low, consistent with the most metal-poor halo stars, but not with the
typical halo abundance pattern at [Fe/H]>-3.0. Our results are broadly
consistent with a galaxy formation model that predicts that massive dwarf
galaxies are the source of the metal-rich component ([Fe/H]>-2.5) of the MW
halo, but we also suggest that the faintest known dwarfs may be the primary
contributors to the metal-poor end of the MW halo metallicity distribution.
