Chemical abundances of the damped Ly alpha systems at z > 1.5

We present chemical abundance measurements for 19 damped Ly alpha systems o bserved with the high-resolution echelle spectrograph (HIRES) on the 10 m W .M. Keck Telescope. We perform a detailed analysis of every system, derivin g ionic column densities for all unblended metal-line transitions. Our prin cipal goal is to investigate the abundance patterns of the damped systems a nd thereby determine the underlying physical processes that dominate their chemical evolution. We place particular emphasis on gauging the relative im portance of two complementary effects often invoked to explain the damped L y alpha abundances (1) nucleosynthetic enrichment from Type II supernovae a nd (2) an interstellar medium-like (ISM-like) dust-depletion pattern. Simil ar to the principal results of Lu et al., our observations lend support bot h for dust depletion and Type II supernova (SN) enrichment. Specifically, t he observed overabundance of Zn/Fe and underabundance of Ni/Fe relative to solar abundances suggest significant dust depletion within the damped Lya s ystems. Meanwhile, the relative abundances of Al, Si, and Cr versus Fe are consistent with both dust depletion and Type II supernova enrichment. Our m easurements of Ti/Fe and the Mn/Fe measurements from Lu et al., however, ca nnot be explained by dust depletion and indicate an underlying Type II SN p attern. Finally, the observed values of [S/Fe] are inconsistent with the co mbined effects of dust depletion and the nucleosynthetic yields expected fo r Type II supernovae. This last result emphasizes the need for another phys ical 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 surv eys 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 f rom a small sample of high-z damped systems.