QSO absorption-line systems and early chemical evolution

Heavy-element absorption-line systems in spectra of QSOs represent unique probes of gas at high redshifts, An understanding of these systems will be important for theories of galaxy formation and large-scale structure. By using spectroscopic studies to obtain abundances in QSO absorption-line systems at different redshifts, it is possible to observe 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 absorbing gas in which the heavy elements are predominantly those produced 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 present state of abundance measurements in damped Lyman-alpha systems, as well as recent results of studies of Galactic halo-star and interstellar 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 QSO absorption-line systems with redshift. The range of overall abundances (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, measurable tracers of supernova activity and dust grain formation in the absorber population are identified. While the available data for O and 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 only upper limits. Future observations of O, N, Ca, Ti, and Cr should shed new light on the origin of the elements and of grains in the absorbers, Observed cosmic time scales for elemental buildup can lead to specific predictions for the typical rates of Type I and Type II SNe as a function of redshift.