The Rapid Decline in Metallicity of Damped Ly-alpha Systems at z~5
  We present evidence that the cosmological mean metallicity of neutral atomic
hydrogen gas shows a sudden decrease at z>4.7 down to <Z>=-2.03(+0.09-0.11),
which is 6 sigma deviant from that predicted by a linear fit to the data at
lower redshifts. This measurement is made possible by the chemical abundance
measurements of 8 new damped Ly-alpha (DLA) systems at z>4.7 observed with the
Echellette Spectrograph and Imager on the Keck II telescope, doubling the
number of measurements at z>4.7 to 16. Possible explanations for this sudden
decrease in metallicity include a change in the physical processes that enrich
the neutral gas within disks, or an increase of the covering factor of neutral
gas outside disks due to a lower ultra-violet radiation field and higher
density at high redshift. The later possibility would result in a new
population of presumably lower metallicity DLAs, with an increased contribution
to the DLA population at higher redshifts resulting in a reduced mean
metallicity. Furthermore, we provide evidence of a possible decrease at z>4.7
in the comoving metal mass density of DLAs, which is flat out to z~4.3. Such a
decrease is expected, as otherwise most of the metals from star-forming
galaxies would reside in DLAs by z~6. While the metallicity is decreasing at
high redshift, the contribution of DLAs to the total metal budget of the
universe increases with redshift, with DLAs at z~4.3 accounting for ~20% as
many metals as produced by Lyman break galaxies.
