QSO ABSORPTION-LINE SYSTEMS AND EARLY CHEMICAL EVOLUTION

Heavy-element absorption-line systems in spectra of QSOs represent uni que probes of gas at high redshifts, An understanding of these systems will be important for theories of galaxy formation and large-scale st ructure. By using spectroscopic studies to obtain abundances in QSO ab sorption-line systems at different redshifts, it is possible to observ e clues to the nucleosynthetic history of objects, at a large range in redshift, that are presumably at different points in their evolution. In principle, one could imagine using abundance ratios to identify ab sorbing gas in which the heavy elements are predominantly those produc ed in Type II SNe (as are the Galactic halo and oldest disk stars) and those which have a mixture of the products of Type I and Type II SNe (like the gas and young stars of the Galactic disk), We review the pre sent state of abundance measurements in damped Lyman-alpha systems, as well as recent results of studies of Galactic halo-star and interstel lar abundances. We also briefly discuss the results of imaging studies of fields around QSOs showing absorption by intervening gas, and the results of statistical analyses of the change in number of observed QS O absorption-line systems with redshift. The range of overall abundanc es (10(-0.5) to 10(-3) solar) found in the absorbers (at redshifts of 0.6-3.4) overlaps the range found in Galactic halo stars. Specific, me asurable tracers of supernova activity and dust grain formation in the absorber population are identified. While the available data for O an d S are sparse, there is evidence for systems with [Si/Fe]similar to+0 .4, a distinguishing characteristic of Galactic halo-star abundances. Dust-free gas with halo-star abundances and solar-metallicity gas with a Galactic cold-cloud depletion pattern are excluded by existing data . Additional observations of absorption-line systems with 10(19)<N(H I )<10(22) to obtain abundances of Al, Si, S, Mn, Fe, and Zn are needed. High spectral resolution is critical so that H I and H Ir regions may be identified and separated; otherwise the derived abundances are onl y upper limits. Future observations of O, N, Ca, Ti, and Cr should she d new light on the origin of the elements and of grains in the absorbe rs, Observed cosmic time scales for elemental buildup can lead to spec ific predictions for the typical rates of Type I and Type II SNe as a function of redshift.