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We report new observations of Zn II and Cr II absorption lines in 10 damped Lyman α systems (DLAs), mostly at redshift z abs > ∼ 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 Lyman α 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 two orders of magnitude, in the metallicities we measure at essentially the same redshifts. We propose that damped Lyman α 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 Lyman α systems is in agreement with expectations based on the consumption of H I gas implied by the recent measurements of Ω DLA by Storrie-Lombardi et al. (1996a), and with the metal ejection rates in the universe at these epochs deduced by Madau (1996) 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 Lyman α 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

Measurements of Zn and Cr abundances in 18 damped Lyman α systems (DLAs) at absorption redshifts z abs = 0.692 − 3.390 (but mostly between z ≃ 2 and 3) show that metals and dust are much less abundant in high redshift galaxies than in the Milky Way today. Typically, [Zn/H] ≃ −1.2; as Zn tracks Fe closely in Galactic stars of all metallicities and is only lightly depleted onto interstellar grains, we conclude that the overall degree of metal enrichment of damped Lyman α galaxies ≈ 13.5 Gyr ago (H 0 = 50 km s −1 Mpc −1 , q 0 = 0.05) was ∼ 1/15 solar.Values of [Cr/Zn] span the range from ≃ 0 to < ∼ − 0.65 which we interpret as evidence for selective depletion of Cr onto dust in some DLAs. On average Cr and other refractory elements are depleted by only a factor of ≈ 2, significantly less than in local interstellar clouds. We propose that this reflects an overall lower abundance of dust-which may be related to the lower metallicities, likely higher temperature of the ISM, and higher supernova rates in these young galaxies-rather than an "exotic" composition of dust grains.Combining a metallicity Z DLA ≃ 1/15Z ⊙ with a dust-to-metals ratio ≈ 1/2 of that in local interstellar clouds, we deduce that the "typical" dust-to-gas ratio in damped Lyman α galaxies is ≈ 1/30 of the Milky Way value. This amount of dust will introduce an extinction at 1500 Å of only A 1500 ≈ 0.1 in the spectra of background QSOs. Similarly, we expect little reddening of the broad spectral energy distribution of the high-z field galaxies now being found routinely by deep imaging surveys. Even such trace amounts of dust, however, can explain the weakness of Lyman α emission from star-forming regions. We stress the approximate nature of such general statements; in reality, the range of metallicities and dust depletions encountered indicates that some sight-lines -3through high-redshift galaxies may be essentially dust-free, while others could suffer detectable extinction.Finally, we show that, despite claims to the contrary, these conclusions are not inconsistent with recent high resolution observations of DLAs with the Keck telescope. We point out that the star-formation histories of high-z galaxies are not necessarily the same as that of the Milky Way and that, if depletions of some elements onto dust are not taken into account correctly, it is possible to misinterpret the clues to early nucleosynthesis provided by non-solar element ratios.