On ionization effects and abundance ratios in damped Lyman alpha systems

Abstract

The similarity between observed velocity structures of Al III and singly ionized species in damped Lyman-alpha systems (DLAs) suggests the presence of ionized gas in the regions where most metal absorption lines are formed. A simplified model consisting of Region 1) a plane-parallel ionization bounded region illuminated by an internal radiation field, and Region 2) a neutral region with a negligible metal content is considered. We calculate photoionization equilibrium models for region 1, and constrain the ionization parameter by the observed N(Al III)/N(Si II) column density ratio. Under these conditions we find that ionization effects are important. If these effects are taken into account, the element abundance ratios in DLAs are quite consistent with those observed in Milky Way stars and in H II regions of local low-metallicity blue compact dwarf galaxies. In particular we cannot exclude the same primary N origin in both DLAs and metal-poor galaxies. No depletion of heavy elements on dust grains needs to be invoked, although our models do not exclude the presence of little depletion. Although highly simplified and relying on the strong assumption of a significantly lower metal content in region 2, our model appears to be supported by recent data on a local DLA and it is not in contradiction with the current knowledge on high redshift DLAs. If correct, it offers a clear simplification in the understanding of heavy element abundance ratios in DLAs and their comparison with the local Universe.