Chemical enrichment at high redshifts: Understanding the nature of damped Ly alpha systems in hierarchical models

We use cosmological hydrodynamical simulations including star formation and metal enrichment to study the evolution of the chemical properties of gala xy-like objects at high redshift in the range 0.25 < z < 2.35 in a hierarch ical clustering scenario. We find that as the galactic objects are assemble d, their gaseous components exhibit neutral hydrogen column densities with abundances and scatter comparable to those observed in damped Ly alpha syst ems (DLAs). The unweighted mean of abundance ratios and least-square linear regressions through the simulated DLAs yield intrinsic metallicity evoluti on for [Zn/H] and [Fe/H] consistent with results obtained from similar anal yses of available observations. Our model statistically reproduces the mild evolution detected in the metallicity of the neutral hydrogen content of t he universe, given by mass-weighted means, if observational constraints are considered (as suggested in 1998 by Boissee and co-workers). For the alpha -elements in the simulated DLAs, we find neither enhancement nor dependenc e on metallicity. Our results support the hypotheses that DLAs trace a vari ety of galactic objects with different formation histories and that both Ty pe I and Type II supernovae are contributing to the chemical enrichment of the gas component, at least since z approximate to 2. This study indicates that DLAs could be understood as the building blocks that merged to form cu rrent normal galaxies within a hierarchical clustering scenario.