  High redshift DLA systems 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 abundance, and
from DLA absorbers with small dust depletion and negligible HII contamination.
In particular, in two DLA systems nitrogen is detected and at remarkably high
levels (Vladilo et al. 1995, Molaro et al. 1995, Green et al. 1995, Kulkarni et
al. 1996). 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 in the framework of a chemical
evolution model assuming short but intense bursts of star formation, which in
turn trigger enriched galactic winds, and a primary origin for nitrogen in
massive stars. Such a model is the most successful in describing the chemical
abundances of dwarf irregular galaxies and in particular of the peculiar galaxy
IZw18. 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.
