On ionisation effects and abundance ratios in damped Lyman-alpha systems

The similarity between observed velocity structures of Al III and singly io nised species in damped Ly alpha systems (DLAs) suggests the presence of io nised gas in the regions where most metal absorption lines are formed. To e xplore the possible implications of ionisation effects we construct a simpl ified two-region model for DLAs consisting of an ionisation bounded region with an internal radiation field and a neutral region with a lower metal co ntent. Within this framework we find that ionisation effects are important. If taken into account, the element abundance ratios in DLAs are quite cons istent with those observed in Milky Way stars and in metal-poor H II region s in blue compact dwarf galaxies. In particular we cannot exclude the same primary N origin in both DLAs and metal-poor galaxies. From our models no d ust depletion of heavy elements needs to be invoked; little depletion is ho wever not excluded.