Clues to the nature of damped Lyman alpha systems from chemical evolution models

Abstract

The evolution of the metallicity of damped Lyman alpha systems (DLAs) is investigated in order to understand the nature of these systems. The observational data on chemical abundances of DLAs are analysed with robust statistical methods, and the abundances are corrected for dust depletion. The results of this analysis are compared to predictions of several classes of chemical evolution models: one-zone dwarf galaxy models, multizone disk models, and chemodynamical models representing dwarf galaxies. We compare the observational data on the [alpha/Fe] and [N/alpha] ratios to the predictions from the models. In DLAs, these ratios are only partially reproduced by the dwarf galaxy one-zone model and by the disk model. On the other hand, the chemodynamical model for dwarf galaxies reproduces the properties of nearly all DLAs. We derive the formation epoch of dwarf galaxies, and we find that dwarf galaxies make a significant contribution to the total neutral gas density in DLAs, and that this contribution is more important at high redshifts (z > 2-3). We propose a scenario in which the DLA population is dominated by dwarf galaxies at high redshifts and by disks at lower redshifts. We also find that Lyman Break Galaxies (LBGs) may constitute a sequence rather than present a sharp dichotomy between the two populations. We also arise the possibility that we could be missing a whole population of high HI density column objects, with metallicities intermediate between those of DLAs and LBGs. Finally, we discuss the possibility that relying only on the observations of DLAs could lead to an underestimate of the metal content of the high redshift Universe.Comment: 23 pages, 19 figures. Accepted for publication in MNRA