Ultrafaint Dwarfs - Star Formation and Chemical Evolution in the
  Smallest Galaxies
  In earlier work we showed that a dark matter halo with a virial mass of
$10^7$ M$_\odot$ can retain a large percentage of its baryons in the face of
the pre-ionization phase and supernova explosion from a 25$M_\odot$ star. Here
we expand on the results of that work, investigating the star formation and
chemical evolution of the system beyond the first supernova. In a galaxy with a
mass $M_{vir} = 10^7$M$_\odot$, sufficient gas is retained by the potential for
a second period of star formation to occur. The impact of a central explosion
is found to be much stronger than that of an off-centred explosion both in
blowing out the gas and in enriching it, as in the off-centered case most of
the supernova energy and metals escape into the IGM. We model the star
formation and metallicity given the assumption that stars form for 100, 200,
400 and 600 Myr and discuss the results in the context of recent observations
of very low mass galaxies. We show that we can account for most features of the
observed relationship between [$\alpha$/Fe] and [Fe/H] in ultra-faint dwarf
galaxies with the assumption that the systems formed at a low mass, rather than
being remnants of much larger systems.
