The Metallicity of High-Redshift Galaxies: The Abundance of Zinc in 34 Damped Lyα Systems from z = 0.7 to 3.4

We report new observations of Zn II and Cr II absorption lines in 10 damped Lyα systems (DLAs), mostly at redshift zabs ≳ 2.5. By combining these results with those from our earlier survey and other recent data, we construct a sample of 34 measurements (or upper limits) of the Zn abundance relative to hydrogen [Zn/H]; the sample includes more than one-third of the total number of DLAs known. The plot of the abundance of Zn as a function of redshift reinforces the two main findings of our previous study. (1) Damped Lyα systems are mostly metal poor, at all redshifts sampled; the column density-weighted mean for the whole data set is [Zn/H] = -1.13 ± 0.38 (on a logarithmic scale), or approximately 1/13 of solar. (2) There is a large spread, by up to 2 orders of magnitude, in the metallicities we measure at essentially the same redshifts. We propose that damped Lyα systems are drawn from a varied population of galaxies of different morphological types and at different stages of chemical evolution, supporting the idea of a protracted epoch of galaxy formation. At redshifts z ≳ 2, the typical metallicity of the damped Lyα systems is in agreement with expectations based on the consumption of H I gas implied by the recent measurements of ΩDLA by Storrie-Lombardi et al., and with the metal ejection rates in the universe at these epochs deduced by P. Madau from the ultraviolet luminosities of high-redshift galaxies revealed by deep imaging surveys. There are indications in our data for an increase in the mean metallicity of the damped Lyα systems from z > 3 to ≈ 2, consistent with the rise in the comoving star formation rate indicated by the relative numbers of U and B drop-outs in the Hubble Deep Field. Although such comparisons are still tentative, it appears that these different avenues for exploring the early evolution of galaxies give a broadly consistent picture. At redshifts z < 1.5, DLAs evidently do not exhibit the higher abundances expected from a simple, closed-box model of global chemical evolution, although the number of measurements is still very small. We speculate that this may be due to an increasing contribution of low surface brightness galaxies to the cross section for damped Lyα absorption and to the increasing dust bias with decreasing redshift proposed by S. M. Fall and collaborators. However, more DLAs at intermediate redshifts need to be identified before the importance of these effects can be assessed quantitatively. The present sample is sufficiently large for a first attempt at constructing the metallicity distribution of damped Lyα systems and comparing it with those of different stellar populations of the Milky Way. The DLA abundance histogram is both broader and peaks at lower metallicities than those of either thin or thick disk stars. At the time when our Galaxy's metal enrichment was at levels typical of DLAs, its kinematics were closer to those of the halo and bulge than a rotationally supported disk. This finding is at odds with the proposal that most DLAs are large disks with rotation velocities in excess of 200 km s-1, based on the asymmetric profiles of absorption lines recorded at high spectral resolution. Observations of the familiar optical emission lines from H II regions, which are within reach of near-infrared spectrographs on 8-10 m telescopes, may help resolve this discrepancy.