IONIZED GAS IN DAMPED Lyα SYSTEMS AND ITS EFFECTS ON ELEMENTAL ABUNDANCE STUDIES

摘要

Recent high-resolution observations of metal absorption lines in high-redshift damped Lyα systems have shown that AI III, a tracer of moderately ionized gas, very often has a velocity structure indistinguishable from that of low-ionization gas. Regions of ionized and neutral hydrogen in these systems are likely cospatial. The higher ionization Si Iv and C Iv absorption shows a much weaker or nonexistent correlation with the low-ionization material, implying that the regions traced by AI III are photoionized by a soft (stellar) spectrum, by a hard (power- law) spectrum with a very low ionization parameter, or a combination of both. We discuss the ionization of the damped Lyα systems and use photoionization equilibrium models to make quantitative estimates of its effects C on abundance studies in these systems. We show that ionization effects may be large enough to account for the observed dispersion in absolute metal abundances in damped Lyα systems, causing systematically higher abun- dances in lower column density systems. The observed Si~+/Fe~+ and Zn~+/Cr~+ ratios may systematically overestimate the intrinsic Si/Fe and Zn/Cr ratios, respectively, if ionized gas is present in these systems, thereby mimicking the effects of α-element enrichment or dust depletion.