Metal Abundances at [FORMULA][F]z<1.5[/F][/FORMULA]: Fresh Clues to the Chemical Enrichment History of Damped [CLC]Ly[/CLC]α Systems

Abstract

We explore the redshift evolution of the metal content of damped Lyα systems (DLAs) with new observations of four absorbers at z<1.5; together with other recently published data, there is now a sample of 10 systems at intermediate redshifts for which the abundance of Zn has been measured. The main conclusion is that the column density-weighted mean metallicity, [Zn/H]=-1.03±0.23 (on a logarithmic scale), is not significantly higher at z<1.5 than at earlier epochs, despite the fact that the comoving star formation rate density of the universe was near its maximum value at this redshift. Gas of high column density and low metallicity dominates the statistics of present samples of DLAs at all redshifts. For three of the four DLAs, our observations include absorption lines of Si, Mn, Cr, Fe, and Ni, as well as Zn. We argue that the relative abundances of these elements are consistent with a moderate degree of dust depletion that, once accounted for, leaves no room for the enhancement of the α elements over iron seen in metal-poor stars in the Milky Way. This is contrary to previous assertions that DLAs have been enriched solely by Type II supernovae, but it can be understood if the rate of star formation in the systems studied proceeded more slowly than in the early history of our Galaxy. These results add to a growing body of data pointing to the conclusion that known DLAs do not trace the galaxy population responsible for the bulk of star formation. Possible reasons are that sight lines through metal-rich gas are systematically underrepresented, because the background QSOs are reddened, and that the most actively star-forming galaxies are also the most compact, presenting too small a cross-section to have been probed yet with the limited statistics of current samples.