Early stages of nitrogen enrichment in galaxies: Clues from measurements in damped Lyman <i>α</i>systems

Abstract

We present 4 new measurements of nitrogen abundances and one upper limit in damped Ly α absorbers (DLAs) obtained by means of high resolution ( km s-1) UVES/VLT spectra. In addition to these measurements we have compiled data from all DLAs with measurements of nitrogen and α-capture elements (O, S or Si) available in the literature, including all HIRES/Keck and UVES/VLT data for a total of 32 systems, i.e. the largest sample investigated so far. We find that [ N/α] ratios are distributed in two groups: 75% of the DLAs show a mean value of with a scatter of 0.17 dex, while the remaining 25% shows ratios clustered at with an even lower dispersion of 0.05 dex. The high plateau is consistent with the one observed in metal-poor regions of blue compact dwarf (BCD) galaxies (), while the [ values are the lowest ever observed in any astrophysical site. These low [ N/α] ratios are real and are not due to ionization effects. They provide crucial evidence against the primary production of N by massive stars as being responsible for the plateau at -0.7 dex observed in DLAs and BCD galaxies. The transition between the low-N () and high-N ([ ) DLAs occurs at a nitrogen abundance of , suggesting that the separation may result from some peculiarity of the nitrogen enrichment history. The values and their low dispersion are consistent with a modest production of primary N in massive stars; however, due to the limited sample, specially for the low-N DLAs, we cannot exclude a primary origin in intermediate mass stars as responsible for the low N abundances observed.