Metallicity Evolution of Damped Lyman-alpha Systems out to z~5
  We present chemical abundance measurements for 47 damped Lyman-alpha systems
(DLAs), 30 at z>4, observed with the Echellette Spectrograph and Imager and the
High Resolution Echelle Spectrometer on the Keck telescopes. HI column
densities of the DLAs are measured with Voigt profile fits to the Lyman-alpha
profiles, and we find an increased number of false DLA identifications with
SDSS at z>4 due to the increased density of the Lyman-alpha forest. Ionic
column densities are determined using the apparent optical depth method, and we
combine our new metallicity measurements with 195 from previous surveys to
determine the evolution of the cosmic metallicity of neutral gas. We find the
metallicity of DLAs decreases with increasing redshift, improving the
significance of the trend and extending it to higher redshifts, with a linear
fit of -0.22+-0.03 dex per unit redshift from z=0.09-5.06. The metallicity
'floor' of ~1/600 solar continues out to z~5, despite our sensitivity for
finding DLAs with much lower metallicities. However, this floor is not
statistically different from a steep tail to the distribution. We also find
that the intrinsic scatter of metallicity among DLAs of ~0.5 dex continues out
to z~5. In addition, the metallicity distribution and the alpha/Fe ratios of
z>2 DLAs are consistent with being drawn from the same parent population with
those of halo stars. It is therefore possible that the halo stars in the Milky
Way formed out of gas that commonly exhibits DLA absorption at z>2.
