Early stages of nitrogen enrichment in galaxies: Clues from measurements in damped Lyman 

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 ($FWHM \simeq 7$ 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 $\rm [N/\alpha] = -0.87$ with a scatter of 0.17 dex, while the remaining 25% shows ratios clustered at $\rm [N/\alpha] = -1.45$ with an even lower dispersion of 0.05 dex. The high $\rm [N/\alpha] \simeq -0.9$ plateau is consistent with the one observed in metal-poor \ion{H}{ii} regions of blue compact dwarf (BCD) galaxies ($\rm [N/\alpha] =-0.73 \pm 0.13$), while the [ $\rm N/\alpha] \simeq -1.5$ 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 $-0.9/$ -0.7 dex observed in DLAs and BCD galaxies. 
 The transition between the low-N ($\rm [N/\alpha]\simeq-1.5$) and high-N ([ $\rm N/\alpha] \simeq -0.9$) DLAs occurs at a nitrogen abundance of $\rm [N/H] \simeq -2.8$, suggesting that the separation may result from some peculiarity of the nitrogen enrichment history. The $\rm [N/\alpha ] \simeq -1.5$ 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.