Nitrogen Abundances in Damped Lyα Galaxies

Nitrogen abundances have been derived in Damped Ly alpha (DLA) galaxies atZabs = 2.309, 2.827 and 3.025 toward the QSOs 0100+1300, 1425+6039 and0347-3819 respectively. The behaviour of nitrogen relative to iron-peak andalpha-elements has been investigated by considering all the extant NIdeterminations for a total of 9 DLA galaxies. We have estimated the fraction ofiron locked into dust grains to convert the observed [N/Fe] ratios into overall(dust plus gas) relative abundances, [N/Fe]corr. The ratios [N/alpha] have beenmostly determined by using sulphur as a tracer of alpha-elements which isunaffected by dust. The [N/Fe] and [N/alpha] ratios show high dispersions, ofone order of magnitude or more, which have no equivalent in otherelement-to-element ratios in DLAs. The lowest values of the [N/Fe]corr and[N/alpha] ratios are at variance with the values measured in Galactic halostars of similar metallicity suggesting that part of the DLA galaxies do notfollow the chemical evolution of the Milky Way. The DLA nitrogen abundances andtheir dispersion show some similarities with those observed in dwarf galaxies.The behaviour of nitrogen abundance ratios can be ascribed, in general to thedelayed release of nitrogen in the course of evolution. However it is difficultto conciliate this interpretation with the lowest [N/alpha] values measured,since an expected enhancement of alpha-elements respect to the iron-peakelements is not observed simultaneously in these DLA galaxies. In two cases,relatively high [N/alpha] values are observed which require also a more complexchemical evolution to be explained.Comment: 29 pages including 5 tables and figure captions,LaTeX, 8 figures, ApJ accepte