Chemical Evolution at High Redshift

Abstract

Observations of high-redshift Damped Ly—α systems (DLA) suggest that the relative abundances of elements might be roughly solar, although with absolute abundances of more than two orders of magnitude below solar. The result comes from observations of the [SII/ZnII] ratio, which is a reliable diagnostic of the true abundances, and from DLA absorbers with a small dust depletion and negligible HII contamination. In particular, in two DLA systems nitrogen is detected at remarkably high levels (Vladilo et al. 1995, Molaro et al. 1996, Green et al. 1995). Here we compare the predictions from chemical evolution models of galaxies of different morphological type with the abundances and abundance ratios derived for such systems. We conclude that solar ratios and relatively high nitrogen abundances can be obtained only in the framework of a chemical evolution model relative to a galaxy having one of its first intense bursts of star formation, which, in turn, triggers O-enriched galactic winds, together with a primary origin for nitrogen in massive stars. This model is the most successful in describing the chemical evolution of dwarf irregular galaxies and in particular of the peculiar galaxy IZwl8 (Kunth et al. 1995). Thus, solar ratios at very low absolute abundances, if confirmed, seem to favour dwarf galaxies rather than spirals as the progenitors of at least some of the DLA systems.

On the other hand, other three DLA observed by Pettini et al. (1995) and Lipman (1995) are better fitted by models reproducing the solar neighbourhood and therefore are likely to be protospirals.

We conclude by stressing that abundance ratios between elements produced by stars in different mass ranges, such as N/O and α/Fe, represent extremely useful tools to interpret the nature of high-redshift objects.