Early metal enrichment of gas-rich galaxies at z~5
  We present abundance measurements of elements O, C, Si and Fe for three
gas-rich galaxies at z~5 using observations from the Very Large Telescope (VLT)
and the Keck telescope in order to better constrain the early chemical
enrichment of gas-rich galaxies. These galaxies show strong Lyman-{\alpha}
absorption in the spectra of background quasars, with neutral hydrogen column
densities log N${_H{}_I}$(cm$^{-2}$)=20.10$\pm$0.15, 20.10$\pm$0.15, and
20.80$\pm$0.15. Using the undepleted element O, we find the metallicities [O/H]
to be in the range of -2.51 to -2.05 dex. Our study has doubled the existing
sample of measurements of undepleted elements at z>4.5. Combining our
measurements with those from the literature, we find that the
N${_H{}_I}$-weighted mean metallicity of z~5 absorbers is consistent with the
prediction based on z<4.5 DLAs. Thus, we find no significant evidence of a
sudden drop in metallicity at z>4.7 as reported by some prior studies. We also
determine the extent of dust depletion using a combination of both the volatile
element O and the refractory elements Si and/or Fe. Some of the absorbers show
evidence of depletion of elements on dust grains, e.g. low [Si/O] or [Fe/O].
The relative abundances of these absorbers along with other z~5 absorbers from
the literature show some peculiarities, e.g. low [C/O] in several absorbers and
high [Si/O] in one absorber. Finally, we find that the metallicity vs. velocity
dispersion relation of z~5 absorbers may be different from that of
lower-redshift absorbers.
