METAL ENRICHMENT, DUST, AND STAR FORMATION IN GALAXIES AT HIGH REDSHIFTS .1. THE ZETA=2.3091 ABSORBER TOWARD PHL-957

We present the first results from a survey aimed at determining the degree of metal enrichment, the dust content, and the star formation rate of QSO absorption systems with damped Lyα lines. This class of absorbers is thought to arise in the high-redshift progenitors of present-day disk galaxies. We show how measurements of weak absorption lines of Zn II-detected here for the first time in a QSO spectrum - and of Cr II can provide accurate estimates of the metallicity and an indication of the dust-to-gas ratio, while an informative upper limit on the star formation rate can be obtained by searching for weak emission in the core of the damped Lyα line. The zabs = 2.3091 system in PHL 957 has both a low metal abundance, 24 times below solar, and a low dust-to-gas ratio, approximately one order of magnitude lower than values typical of local interstellar clouds. These results suggest that the absorption system arises in a galaxy at an early stage of chemical evolution. The star formation rate is less than ∼30 M⊙ yr-1. These and other parameters are strikingly similar to those of nearby H II galaxies and, if found by future work to be typical of the damped Lyα sample in general, would suggest that disk galaxies at z ≃ 2.5 resembled closely the H II galaxies we see today. Alternatively, the damped Lyα lines may arise predominantly in dwarf galaxies whose properties have not changed significantly from z ≃ 2.5 to the present epoch. Further observations of damped Lyα systems should allow us to distinguish between these two possibilities and offer excellent opportunities for measuring the primordial abundance of deuterium.