Segue 1: An Unevolved Fossil Galaxy from the Early Universe
  We present Magellan/MIKE and Keck/HIRES high-resolution spectra of six red
giant stars in the dwarf galaxy Segue 1. Including one additional Segue 1 star
observed by Norris et al. (2010), high-resolution spectra have now been
obtained for every red giant in Segue 1. Remarkably, three of these seven stars
have metallicities below [Fe/H] = -3.5, suggesting that Segue 1 is the least
chemically evolved galaxy known. We confirm previous medium-resolution analyses
demonstrating that Segue 1 stars span a metallicity range of more than 2 dex,
from [Fe/H] = -1.4 to [Fe/H] = -3.8. All of the Segue 1 stars are
alpha-enhanced, with [alpha/Fe] ~ 0.5. High alpha-element abundances are
typical for metal-poor stars, but in every previously studied galaxy [alpha/Fe]
declines for more metal-rich stars, which is typically interpreted as iron
enrichment from supernova Ia. The absence of this signature in Segue 1
indicates that it was enriched exclusively by massive stars. Other light
element abundance ratios in Segue 1, including carbon-enhancement in the three
most metal-poor stars, closely resemble those of metal-poor halo stars.
Finally, we classify the most metal-rich star as a CH star given its large
overabundances of carbon and s-process elements. The other six stars show
remarkably low neutron-capture element abundances of [Sr/H] < -4.9 and [Ba/H] <
-4.2, which are comparable to the lowest levels ever detected in halo stars.
This suggests minimal neutron-capture enrichment, perhaps limited to a single
r-process or weak s-process synthesizing event. Altogether, the chemical
abundances of Segue 1 indicate no substantial chemical evolution, supporting
the idea that it may be a surviving first galaxy that experienced only one
burst of star formation.
