METAL ENRICHMENT, DUST, AND STAR-FORMATION IN GALAXIES AT HIGH REDSHIFTS .3. ZN AND CR ABUNDANCES FOR 17 DAMPED LYMAN-ALPHA SYSTEMS

We present measurements of the abundances of zinc and chromium in a moderately large sample of damped Lyman-a systems at redshifts between z(abs) = 1.78 and z(abs) = 3.03; the sample includes approximately one-third of the total number of damped QSO absorbers known at present. The weak absorption lines of the two trace elements targeted by our survey are of key diagnostic value. [Zn/H] measures the overall degree of metal enrichment in the high-redshift galaxies giving rise to the damped Lyman-alpha systems, while [Cr/Zn] gives an indication of the extent to which refractory elements have been removed from the gaseous interstellar medium, presumably to form dust. With the moderately high signal-to-noise ratio and spectral resolution achieved in the survey (requiring long integration times on 4 m class telescopes, typically 30,000 s per object), we detect Zn II and Cr II absorption lines in about half the objects observed; in the other cases the 3 sigma detection limits for the equivalent widths of the lines range from 16 to 49 mangstrom. We report three principal results: 1. At redshift z congruent-to 2 the typical metallicity of the damped Lyman-a galaxies, Z(D), is approximately 1/10 of solar. Since this class of QSO absorbers traces the neutral baryon content of the universe at early redshifts, the finding that Z(D) = 1/10 Z. can be interpreted as a measure of the characteristic ''cosmic'' metallicity attained approximately 13.5 Gyr ago (H-0 = 50 km s-1 Mpc-1; q0 = 0.01), provided that the known damped Lyman-alpha systems do not preferentially pick out unevolved gas due to selection effects. 2. Them appears to be a considerable range, spanning between one and two orders of magnitude, in the degree of metal enrichment reached by different damped Lyman-alpha galaxies at essentially the same epoch. Simulations indicate that the wide range of values is unlikely to result from radial abundance gradients similar to those seen in nearby spirals. Both the distribution and the typical metallicity Z(D) that we measure are more in line with the values found in halo, rather than disk, stars in our own Galaxy. Thus, if the damped Lyman-alpha systems arise in the progenitors of present-day large galaxies, as is generally thought, we conclude that at z congruent-to 2 we observe them during a stage prior to the bulk of star formation in the disk, possibly still during the collapse of the halo. Alternatively, more than one population of galaxies, of widely different masses and evolving at different rates, may be responsible for producing damped. Lyman-alpha lines in the spectra of QSOs. 3. Even at the early stages of chemical evolution prevailing at z congruent-to 2, the bulk of a refractory element such as Cr appears to have. been incorporated in dust particles; we deduce a typical dust-to-ps ratio approximately 1/10 of the value encountered in the Galactic interstellar medium (ISM) today. However, gas depletions of refractory elements are substantially less severe than in local interstellar clouds; we suggest that this is probably related to the finding that a smaller fraction of the ISM in these high-redshift galaxies is in the form of dense molecular clouds, where grain growth is thought to occur. There may be a trend between gas depletions and overall metallicity of the ISM which, if confirmed, would make the abundances of refractory elements useful diagnostics of physical conditions in the ISM. As is often the case, the work just completed suggests several further lines of inquiry. We emphasize in particular the potential of (1) extending the abundance measurements to a wider redshift interval and (2) broad-band imaging of the absorbers, for elucidating the nature of the damped Lyman-alpha galaxies and following their chemical evolution from the earliest times to the present epoch.