Ionized Gas in Damped Lyman-alpha Systems and Its Effects on Elemental Abundance Studies

Abstract

Recent high-resolution observations of metal absorption lines in high-redshift damped Ly-alpha systems have shown that Al 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 non-existent correlation with the low ionization material, implying that the regions traced by Al 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 Ly-alpha systems and use photoionization equilibrium models to make quantitative estimates of its effects 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 Ly-alpha systems, causing systematically higher abundances 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 alpha-element enrichment or dust depletion.