The most metal-poor damped Lyman-alpha systems: Insights into chemical
  evolution in the very metal-poor regime
  We present a high spectral resolution survey of the most metal-poor damped
Lyman-alpha absorption systems (DLAs) aimed at probing the nature and
nucleosynthesis of the earliest generations of stars. Our survey comprises 22
systems with iron abundance less than 1/100 solar; observations of seven of
these are reported here for the first time. Together with recent measures of
the abundances of C and O in Galactic metal-poor stars, we reinvestigate the
trend of C/O in the very metal-poor regime and we compare, for the first time,
the O/Fe ratios in the most metal-poor DLAs and in halo stars. We confirm the
near-solar values of C/O in DLAs at the lowest metallicities probed, and find
that their distribution is in agreement with that seen in Galactic halo stars.
We find that the O/Fe ratio in very metal-poor (VMP) DLAs is essentially
constant, and shows very little dispersion, with a mean [<O/Fe>] = +0.39 +/-
0.12, in good agreement with the values measured in Galactic halo stars when
the oxygen abundance is measured from the [O I] 6300 line. We speculate that
such good agreement in the observed abundance trends points to a universal
origin for these metals. In view of this agreement, we construct the abundance
pattern for a typical very metal-poor DLA and compare it to model calculations
of Population II and Population III nucleosynthesis to determine the origin of
the metals in VMP DLAs. Our results suggest that the most metal-poor DLAs may
have been enriched by a generation of metal-free stars; however, given that
abundance measurements are currently available for only a few elements, we
cannot yet rule out an additional contribution from Population II stars.
