High-Resolution Spectroscopy of Extremely Metal-Poor Stars in the Least
  Evolved Galaxies: Leo IV
  We present high-resolution Magellan/MIKE spectroscopy of the brightest star
in the ultra-faint dwarf galaxy Leo IV. We measure an iron abundance of [Fe/H]
= -3.2, adding to the rapidly growing sample of extremely metal-poor stars
being identified in Milky Way satellite galaxies. The star is enhanced in the
alpha elements Mg, Ca, and Ti by ~0.3 dex, very similar to the typical Milky
Way halo abundance pattern. All of the light and iron-peak elements follow the
trends established by extremely metal-poor halo stars, but the neutron-capture
elements Ba and Sr are significantly underabundant. These results are quite
similar to those found for stars in the ultra-faint dwarfs Ursa Major II, Coma
Berenices, Bootes I, and Hercules, suggesting that the chemical evolution of
the lowest luminosity galaxies may be universal. The abundance pattern we
observe is consistent with predictions for nucleosynthesis from a Population
III supernova explosion. The extremely low metallicity of this star also
supports the idea that a significant fraction (>10%) of the stars in the
faintest dwarfs have metallicities below [Fe/H] = -3.0.
