The chemical evolution of the Bootes I ultra-faint dwarf galaxy
  We present chemical abundance measurements of two metal-poor red giant stars
in the ultra-faint dwarf galaxy Bootes I, based on Magellan/MIKE
high-resolution spectra. For Boo I-980, with [Fe/H]=-3.1, we present the first
elemental abundance measurements while Boo I-127, with [Fe/H]=-2.0, shows
abundances in good agreement with previous measurements. Light and iron-peak
element abundance ratios in the two Bootes I stars, as well as those of most
other Boootes I members, collected from the literature, closely resemble those
of regular metal-poor halo stars. Neutron-capture element abundances Sr and Ba
are systematically lower than the main halo trend, and also show a significant
abundance spread. Overall, this is similar to what has been found for other
ultra-faint dwarf galaxies. We apply corrections to the carbon abundances
(commensurate with stellar evolutionary status) of the entire sample and find
21% of stars to be carbon-enhanced metal-poor (CEMP) stars, compared to 13%
without using the carbon correction. We reassess the metallicity distribution
functions (MDF) for the CEMP stars and non-CEMP stars, and confirm earlier
claims that CEMP stars might belong to a different, earlier population.
Applying a set of abundance criteria to test to what extent Bootes I could be a
surviving first galaxy suggests that it is one of the earliest assembled
systems that perhaps received gas from accretion from other clouds in the
system, or from swallowing a first galaxy or building block type object. This
resulted in the two stellar populations observable today.
