On the origin of damped lya systems: A case for low surface brightness galaxies?

We use a model of galaxy disk formation to explore the metallicities, dust content, and neutral gas mass density of damped Lyα (DLyα) absorbers. We find that the [Zn/H] abundance measurements of DLyα systems now available can be reproduced either by a population of low surface brightness (LSB) galaxies forming at redshifts z > 3, whose chemical contents evolve slowly with time and whose star formation rates are described by continuous bursts, or by high surface brightness (HSB) galaxies that form continuously over an interval of z ∼ 0.5-3 (and no higher). Although, in reality, a mixture of galaxy types may be responsible for low-z DLyα systems, our models predict that HSB galaxies form more dust, more rapidly, than LSB galaxies and that HSB galaxies may therefore obscure background QSOs and not give rise to DLyα lines, as suggested by other researchers. Significantly, we find that the rate at which HSB disks consume neutral gas is too fast to explain the observed evolution in the neutral gas mass density with redshift and that the consumption of hydrogen by LSB galaxies fits the data better. This further suggests that LSB disks may dominate the DLyα population at high redshift.