Metal enrichment, dust, and star formation in galaxies at high redshifts. 3: Zn and CR abundances for 17 damped Lyman-alpha systems

Abstract

We present measurements of the abundances of zinc and chromium in a moderately large sample of damped Lyman-alpha systems at redshifts between zabs = 1.78 and zabs = 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 mA. We report three principal results. 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.