Nitrogen abundances in damped Ly alpha galaxies

Nitrogen lines of the N I lambda 1134 and lambda 1200 multiplets in the dam ped Ly alpha (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 similar to 2 x 10(4)) obtained with 4 m class telescopes at ESO (La Silla, Chile) and the Observatorio del Rogu e de los Muchachos (La Palma, Spain). The two N I multiplets offer a consid erable range in oscillator strengths and the possibility of disentangling L ya interlopers. The derived nitrogen abundances for the three damped system s are [N/H] = -2.68 +/- 0.11, -1.57 +/- 0.09, -2.07 +/- 0.13, respectively. The behavior of nitrogen relative to iron-peak and alpha elements has been investigated by considering all the extant N I determinations for a total of nine DLA galaxies. We have estimated the fraction of iron locked into du st grains to convert the observed [N/Fe] ratios into overall (dust plus gas ) relative abundances, [N/Fe](corr). The ratios [N/alpha] have been mostly determined by using sulphur as a tracer of a elements that is unaffected by dust. The [N/Fe] and [N/alpha] ratios show high dispersions, of 1 order of magnitude or more, which have no equivalent in other element-to-element ra tios in DLAs. The lowest values of the [N/Fe](corr) and [N/alpha] ratios ar e at variance with the values measured in Galactic halo stars of similar me tallicity, suggesting that part of the DLA galaxies do not follow the chemi cal evolution of the Milky Way. The DLA nitrogen abundances and their dispe rsion show some similarities with those observed in dwarf galaxies. Compari son with chemical evolution models shows that the lowest [N/Fe](corr) and [ N/alpha] DLA values are close to what would be expected for a pure secondar y origin of nitrogen, whereas higher values are mostly consistent with a pr imary component. The behavior 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/alpha] values measured, since an expected enhancement of alpha elements with respect to the iron-peak elements is not observed simultaneously in th ese DLA galaxies. In two cases, relatively high [N/alpha] values are observ ed that require also a more complex chemical evolution to be explained.