ar X iv : a st ro - p h / 98 07 08 2 v 1 8 J ul 1 99 8 Nitrogen abundances in Damped Ly α galaxies

Nitrogen lines of the NI 1134Å and 1200Å multiplets in the Damped Lyα (DLA) galaxies at z abs = 2.309, 2.827 and 3.025 toward the QSOs 0100+1300, 1425+6039 and 0347−3819 respectively, have been detected by means of high resolution spectra (R ∼ 2 × 10 4) obtained with 4-m class telescopes at ESO (La Silla, Chile) and ORM (La Palma, Spain). The two NI multiplets offer a considerable range in oscillator strengths and the possibility of disentangling Lyα interlopers. The derived nitrogen abundances for the three damped systems are [N/H]= –2.68±0.11, –1.57±0.09, –2.07±0.13, respectively. The behaviour of nitrogen relative to iron-peak and α-elements has been investigated by considering all the extant NI determinations for a total of 9 DLA galaxies. We have estimated the fraction of iron locked into dust grains to convert the observed [N/Fe] ratios into overall (dust plus gas) relative abundances, [N/Fe] corr. The ratios [N/α] have been mostly determined by using sulphur as a tracer of α-elements which is unaffected by dust. The [N/Fe] and [N/α] ratios show high dispersions, of one order of magnitude or more, which have no equivalent in other element-to-element ratios in DLAs. The lowest values of the [N/Fe] corr and [N/α] ratios are at variance with the values measured in Galactic halo stars of similar metallicity, suggesting that part of the DLA galaxies do not follow the chemical evolution of the Milky Way. The DLA nitrogen abundances and their dispersion show some similarities with those observed in dwarf galaxies. Comparison with chemical evolution models show that the lowest [N/Fe] corr and [N/α] DLA values are close to what would be expected for a pure secondary origin of nitrogen, while higher values are mostly consistent with a primary component. The behaviour of nitrogen abundance ratios can be ascribed, in general, to the delayed release of nitrogen in the course of evolution. However it is difficult to conciliate this interpretation with the lowest [N/α] values measured, since an expected enhancement of α-elements respect to the iron-peak elements is not observed simultaneously in these DLA galaxies. In two cases, relatively high [N/α] values are observed which require also a more complex chemical evolution to be explained.