THE METALLICITY OF HIGH-REDSHIFT GALAXIES - THE ABUNDANCE OF ZINC IN 34 DAMPED LY-ALPHA SYSTEMS FROM Z = 0.7 TO 3.4

We report new observations of Zn II and Cr II absorption lines in 10 d amped Ly alpha systems (DLAs), mostly at redshift z(abs) greater than or similar to 2.5. By combining these results with those from our earl ier survey and other recent data, we construct a sample of 34 measurem ents (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 reinf orces the two main findings of our previous study. (1) Damped Ly alpha systems are mostly metal poor, at all redshifts sampled; the column d ensity-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 i s a large spread, by up to 2 orders of magnitude, in the metallicities we measure at essentially the same redshifts. We propose that damped Ly alpha systems are drawn from a varied population of galaxies of dif ferent morphological types and at different stages of chemical evoluti on, supporting the idea of a protracted epoch of galaxy formation. At redshifts z greater than or similar to 2, the typical metallicity of t he damped Ly alpha systems is in agreement with expectations based on the consumption of H I gas implied by the recent measurements of Omega (DLA) 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 surve ys. There are indications in our data for an increase in the mean meta llicity of the damped Ly alpha systems from z > 3 to approximate to 2, consistent with the rise in the comoving star formation rate indicate d by the relative numbers of U and B drop-outs in the Hubble Deep Fiel d. Although such comparisons are still tentative, it appears that thes e different avenues for exploring the early evolution of galaxies give a broadly consistent picture. At redshifts z<1.5, DLAs evidently do n ot exhibit the higher abundances expected from a simple, closed-box mo del of global chemical evolution, although the number of measurements is still very small. We speculate that this may be due to an increasin g contribution of low surface brightness galaxies to the cross section for damped Ly alpha absorption and to the increasing dust bias with d ecreasing redshift proposed by S. M. Fall and collaborators. However, more DLAs at intermediate redshifts need to be identified before the i mportance of these effects can be assessed quantitatively. The present sample is sufficiently large for a first attempt at constructing the metallicity distribution of damped Ly alpha systems and comparing it w ith those of different stellar populations of the Milky Way. The DLA a bundance histogram is both broader and peaks at lower metallicities th an those of either thin or thick disk stars. At the time when our Gala xy's metal enrichment was at levels typical of DLAs, its kinematics we re 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 la rge 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.