Early stages of nitrogen enrichment in galaxies: Clues from measurements in damped Lyman α systems ^*,**

¹ Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34131 Trieste, Italy
² Department of Theoretical Astrophysics, Ioffe Physico-Technical Institute, Politechnicheskaya Str. 26, 194021 St. Petersburg, Russia

Corresponding author: M. Centurión, centurio@ts.astro.it

Received: 13 November 2002
Accepted: 30 January 2003

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.