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

We explore the redshift evolution of the metal content of damped Lyman alphasystems (DLAs) with new observations of four absorbers at z < 1.5 . The mainconclusion is that the column density--weighted mean metallicity, [] =-1.03 +/- 0.23 (on a logarithmic scale), is not significantly higher at z < 1.5than at earlier epochs, despite the fact that the comoving star formation ratedensity of the universe was near its maximum value at this redshift. For threeof 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 elementsare consistent with a moderate degree of dust depletion which, once accountedfor, leaves no room for the enhancement of the alpha-elements over iron seen inmetal poor stars in the Milky Way. This is contrary to previous assertions thatDLAs have been enriched solely by Type II supernovae, but can be understood ifthe rate of star formation in the systems studied proceeded more slowly than inthe early history of our Galaxy. These results add to a growing body of dataall pointing to the conclusion that known DLAs do not trace the galaxypopulation responsible for the bulk of star formation. Possible reasons arethat sight-lines through metal rich gas are systematically underrepresentedbecause the background QSOs are reddened, and that the most actively starforming galaxies are also the most compact, presenting too small across-section to have been probed yet with the limited statistics of currentsamples.Comment: 40 pages, LaTex, 9 Postscript Figures. Accepted for publication in the Astrophysical Journa