Element abundances in high-redshift quasar absorbers offer excellent probes of the chemical enrichment of distant galaxies, and can constrain models for population III and early population II stars. Recent observations indicate that the sub-damped Lyman-alpha (sub-DLA) absorbers are more metal-rich than DLA absorbers at redshifts 04.7. However, only 3 DLAs at z>4.5 and none at z>3.5have "dust-free" metallicity measurements of undepleted elements. We report the first quasar sub-DLA metallicity measurement at z>3.5, from detections of undepleted elements in high-resolution data for a sub-DLA at z=5.0. We obtain fairly robust abundances of C, O, Si, and Fe, using lines outside the Lyman-alpha forest. This absorber is metal-poor, with O/H]=-2.00±0.12, which is 4σ below the level expected from extrapolation of the trend for z<3.5 sub-DLAs. The C/O ratio is 1.8 +0.4 −0.3 times lower than in the Sun. More strikingly, Si/O is 3.2 +0.6 −0.5 times lower than in the Sun, while Si/Fe is nearly (1.2 +0.4 −0.3 times) solar. This absorber does not display a clear alpha/Fe enhancement. Dust depletion mayhave removed more Si from the gas phase than is common in the Milky Way interstellar medium, which may be expected if high-redshift supernovae form more silicate-rich dust. C/O and Si/O vary substantially between different velocity components, indicating spatial variations in dust depletion and/or early stellar nucleosynethesis (e.g., population III star initial mass function). The higher velocity gas may trace an outflow enriched by early stars.

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-α absorption in the spectra of background quasars, with neutral hydrogen column densities log N H I (cm −2 ) = 20.10±0.15, 20.10±0.15, and 20.80±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.

We report Keck/ESI and VLT/UVES observations of three super-damped Lyman-α quasar absorbers with H I column densities log N HI ≥21.7 at redshifts 2 z 2.5. All three absorbers show similar metallicities (∼-1.3 to -1.5 dex), and dust depletion of Fe, Ni, and Mn. Two of the absorbers show supersolar [S/Zn] and [Si/Zn]. We combine our results with those for other DLAs to examine trends between N HI , metallicity, dust depletion. A larger fraction of the super-DLAs lie close to or above the line [X/H]=20.59−log N HI in the metallicity vs. N HI plot, compared to the less gas-rich DLAs, suggesting that super-DLAs are more likely to be rich in molecules. Unfortunately, our data for Q0230-0334 and Q0743+1421 do not cover H 2 absorption lines. For Q1418+0718, some H 2 lines are covered, but not detected. CO is not detected in any of our absorbers. For DLAs with log N HI <21.7, we confirm strong correlation between metallicity and Fe depletion, and find a correlation between metallicity and Si depletion. For super-DLAs, these correlations are weaker or absent. The absorbers toward Q0230-0334 and Q1418+0718 show potential detections of weak Ly-α emission, implying star formation rates of ∼1.6 and ∼0.7 M yr −1 , respectively (ignoring dust extinction).Upper limits on the electron densities from C II * /C II or Si II * /Si II are low, but are higher than the median values in less gas-rich DLAs. Finally, systems with log N HI > 21.7 may have somewhat narrower velocity dispersions ∆v 90 than the less gas-rich DLAs, and may arise in cooler and/or less turbulent gas.

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