On Ionization Effects and Abundance Ratios in Damped Lyα Systems

The similarity between observed velocity structures of Al III and singlyionized species in damped Lyman-alpha systems (DLAs) suggests the presence ofionized gas in the regions where most metal absorption lines are formed. Asimplified model consisting of Region 1) a plane-parallel ionization boundedregion illuminated by an internal radiation field, and Region 2) a neutralregion with a negligible metal content is considered. We calculatephotoionization equilibrium models for region 1, and constrain the ionizationparameter by the observed N(Al III)/N(Si II) column density ratio. Under theseconditions we find that ionization effects are important. If these effects aretaken into account, the element abundance ratios in DLAs are quite consistentwith those observed in Milky Way stars and in H II regions of locallow-metallicity blue compact dwarf galaxies. In particular we cannot excludethe same primary N origin in both DLAs and metal-poor galaxies. No depletion ofheavy elements on dust grains needs to be invoked, although our models do notexclude the presence of little depletion. Although highly simplified andrelying on the strong assumption of a significantly lower metal content inregion 2, our model appears to be supported by recent data on a local DLA andit is not in contradiction with the current knowledge on high redshift DLAs. Ifcorrect, it offers a clear simplification in the understanding of heavy elementabundance ratios in DLAs and their comparison with the local Universe.Comment: Accepted for publication in ApJ (scheduled in march 10, 2001 issue) 20 pages including 5 figures (minor corrections in Table 2