METAL ABUNDANCES AND PHYSICAL CONDITIONS IN TWO DAMPED Lyα SYSTEMS TOWARD HS 1946 + 7658

摘要

Quasar metal absorption systems represent some of the best opportunities to study the properties and evolution of galaxies at high redshifts. We present a study of metal abundances and physical conditions in two damped Lya systems at z = 2.8443 and z = 1.7382 toward the quasar 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 element) 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 saturation problem. We also incorporate recently improved oscillator strengths for a number of important transitions. The z = 2.8443 system is found to have an absolute Fe metallicity of 2.4-2.6 dex below solar, where the large range mainly reflects uncertainties in the ionization corrections. The absolute metallicity of the z = 1.7382 system cannot be determined because its H i column density is unknown. We find an apparent 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 abundance ratios are not likely to be significantly biased by ionization effects. More likely, either these ratios represent the intrinsic abundance ratios in the gas, or the gas actually has solar relative abundances with the observed abundance patterns being produced by dust depletion effects. However, the apparent underabundance of Mn to Fe relative to solar argues against the latter interpretation. Interestingly, the observed relative abundance patterns are similar to those seen in Galactic low-metallicity stars, and they can be explained by theories of heavy-element nucleosynthesis in the early stages of chemical enrichment. Systematic studies of how the absolute metallicity and the relative elemental abundance patterns in damped Lya systems change with redshift will eventually allow us to understand the chemical evolution of galaxies. We contrast the properties of the highly ionized gas absorption and the relative distributions of high-ionization and low-ionization 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.