Chemical abundances of the damped Lyα systems at z>1.5

We present chemical abundance measurements for 19 damped Ly alpha systems observed with the high-resolution echelle spectrograph (HIRES) on the 10 m W.M. Keck Telescope. We perform a detailed analysis of every system, deriving ionic column densities for all unblended metal-line transitions. Our principal goal is to investigate the abundance patterns of the damped systems and thereby determine the underlying physical processes that dominate their chemical evolution. We place particular emphasis on gauging the relative importance of two complementary effects often invoked to explain the damped Ly alpha abundances (1) nucleosynthetic enrichment from Type II supernovae and (2) an interstellar medium-like (ISM-like) dust-depletion pattern. Similar to the principal results of Lu et al., our observations lend support both for dust depletion and Type II supernova (SN) enrichment. Specifically, the observed overabundance of Zn/Fe and underabundance of Ni/Fe relative to solar abundances suggest significant dust depletion within the damped Lya systems. Meanwhile, the relative abundances of Al, Si, and Cr versus Fe are consistent with both dust depletion and Type II supernova enrichment. Our measurements of Ti/Fe and the Mn/Fe measurements from Lu et al., however, cannot be explained by dust depletion and indicate an underlying Type II SN pattern. Finally, the observed values of [S/Fe] are inconsistent with the combined effects of dust depletion and the nucleosynthetic yields expected for Type II supernovae. This last result emphasizes the need for another physical process to explain the damped Lya abundance patterns. We also examine the metallicity of the damped Lya systems both with respect to Zn/H and Fe/H. Our results confirm previous surveys by Pettini and collaborators, i.e., [(Zn/H)] = -1.15 +/- 0.15 dex. In contrast with other damped Ly alpha surveys at z > 1.5, we do not formally observe an evolution of metallicity with redshift, although we stress that this result is based on the statistics from a small sample of high-z damped systems.