Chemical Enrichment in the Faintest Galaxies: the Carbon and Iron
  Abundance Spreads in the Bo\"otes I Dwarf Spheroidal Galaxy and the Segue 1
  System
  We present an AAOmega spectroscopic study of red giant stars in Bootes I,
which is an ultra-faint dwarf galaxy, and Segue 1, suggested to be either an
extremely low-luminosity dwarf galaxy or a star cluster. Our focus is
quantifying the mean abundance and abundance dispersion in iron and carbon, and
searching for distant radial-velocity members, in these systems.
  The primary conclusion of our investigation is that the spread of carbon
abundance in both Bootes I and Segue 1 is large. For Bootes I, 4 of our 16
velocity members have [C/H] < ~-3.1, while 2 have [C/H] > ~-2.3, suggesting a
range of Delta[C/H] ~ 0.8. For Segue 1 there exists a range Delta[C/H] ~ 1.0,
including our discovery of a star with [Fe/H] = -3.5 and [C/Fe] = +2.3, which
is a radial velocity member at a distance of 4 half-light radii from the system
center. The accompanying ranges in iron abundance are Delta[Fe/H] ~ 1.6 for
both Bootes I and Segue 1. For [Fe/H] < -3.0, the Galaxy's dwarf galaxy
satellites exhibit a dependence of [C/Fe] on [Fe/H] which is very similar to
that observed in its halo populations. We find [C/Fe] ~ 0.3 for stars in the
dwarf systems that we believe are the counterpart of the Spite et al. (2005)
``unmixed'' giants of the Galactic halo and for which they report [C/Fe] ~ 0.2,
and which presumably represents the natal relative abundance of carbon for
material with [Fe/H] = -3.0 to -4.0.
  We confirm the correlation between luminosity and both mean metallicity and
abundance dispersion in the Galaxy's dwarf satellites, which extends to at
least as faint as Mv = -5. The very low mean metallicity of Segue 1, and the
high carbon dispersion in Bootes I, consistent with inhomogeneous chemical
evolution in near zero-abundance gas, suggest these ultra-faint systems could
be surviving examples of the very first bound systems.
