METAL ABUNDANCES AND PHYSICAL CONDITIONS IN 2 DAMPED LY-ALPHA SYSTEMSTOWARD HS-1946+7658(1)

Quasar metal absorption systems represent some of the best opportuniti es to study the properties and evolution of galaxies at high redshifts . We present a study of metal abundances and physical conditions in tw o damped Ly alpha systems at z = 2.8443 and z = 1.7382 toward the quas ar HS 1946 + 7658. The analyses are based on a high-resolution (FWHM = 20 km s(-1)), high signal-to-noise ratio (40-80 per resolution elemen t) spectrum of the quasar. Ion column densities are determined from a combination of profile fitting and the apparent optical depth method. Special efforts are exercised to minimize the effects of the line satu ration problem. We also incorporate recently improved oscillator stren gths for a number of important transitions. The z = 2.8443 system is f ound to have an absolute Fe metallicity of 2.4-2.6 dex below solar, wh ere the large range mainly reflects uncertainties in the ionization co rrections. The absolute metallicity of the z = 1.7382 system cannot be determined because its H I column density is unknown. We find an appa rent overabundance of Si to Fe relative to their solar ratio by about a factor of 2 in both systems. We also find an overabundance of Si to Al relative to solar by a factor of 3 in the z = 2.8443 system, and an underabundance of Mn to Fe relative to solar by a factor of 3 in the z = 1.7382 system. Photoionization calculations suggest that these abu ndance ratios are not likely to be significantly biased by ionization effects. More likely, either these ratios represent the intrinsic abun dance ratios in the gas, or the gas actually has solar relative abunda nces with the observed abundance patterns being produced by dust deple tion effects. However, the apparent underabundance of Mn to Fe relativ e to solar argues against the latter interpretation. Interestingly, th e observed relative abundance patterns are similar to those seen in Ga lactic low-metallicity stars, and they can be explained by theories of heavy-element nucleosynthesis in the early stages of chemical enrichm ent. Systematic studies of how the absolute metallicity and the relati ve elemental abundance patterns in damped Ly alpha systems change with redshift will eventually allow us to understand the chemical evolutio n of galaxies. We contrast the properties of the highly ionized gas ab sorption and the relative distributions of high-ionization and low-ion ization gas in the z = 2.8443 system with those observed in the Milky Way, and suggest that the highly ionized gas absorption in the damped system is probably produced by photoionization in low-density gas away from the disk of the absorbing galaxy.