Chemical evolution and nature of damped Lyman α systems

We study the nature of damped Lyman alpha systems (DLAs) by means of a comparison between the observed abundances and models of chemical evolution of galaxies of different morphological type. In particular, we compare for the first time the abundance ratios as functions of metallicity and redshift with dust-corrected data. We have developed detailed models following the evolution of several chemical elements (H, D, He, C, N, O, Ne, Mg, Si, S, Fe, Ni and Zn) for elliptical, spiral and irregular galaxies. Each of the models is calibrated to reproduce the main features of a massive elliptical, the Milky Way and the Large Magellanic Cloud (LMC), respectively. In addition, we also run some models for dwarf irregular starburst galaxies. All the models share the same up to date nucleosynthesis prescriptions but differ in their star formation histories. The role of supernovae of different types (II, Ia) is studied in each galaxy model. Our main conclusions are as follows. (i) When dust depletion is taken into account, most of the claimed alpha/Fe overabundances disappear and DLAs show solar or subsolar abundance ratios. (ii) The majority of DLAs can be explained either by discs of spirals observed at large galactocentric distances or by irregular galaxies, such as the LMC, or by starburst dwarf irregulars observed at different times after the last burst of star formation. (iii) Elliptical galaxies cannot be DLA systems because they reach too high a metallicity at early times and their abundance ratios show overabundances of alpha-elements relative to Fe over a large range of [Fe/H]. (iv) The observed neutral gas cosmic evolution is compared with our predictions but no firm conclusions can be drawn in the light of the available data.