We present 4 new measurements of nitrogen abundances and one upper limit in damped Ly \alpha 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 \alpha -capture elements ( O , S or Si ) available in the literature , including all HIRES/Keck and UVES/VLT data for a total of 33 systems , i.e .
the largest sample investigated so far .
We find that [ N/ \alpha ] ratios are distributed in two groups : 75 % of the DLAs show a mean value of [ N/ \alpha ] = –0.87 with a scatter of 0.16 dex , while the remaining 25 % shows ratios clustered at [ N/ \alpha ] = –1.45 with an even lower dispersion of 0.05 dex .
The high [ N/ \alpha ] plateau is consistent with the one observed in metal-poor H ii regions of blue compact dwarf ( BCD ) galaxies ( [ N/ \alpha ] = –0.73 \pm 0.13 ) , while the [ N/ \alpha ] \simeq –1.5 values are the lowest ever observed in any astrophysical site .
These low [ N/ \alpha ] ratios are real and not due to ionization effects .
They provide a crucial evidence against the primary production of N by massive stars as responsible for the plateau at –0.9/–0.7 dex observed in DLAs and BCD galaxies .
The transition between the low-N ( [ N/ \alpha ] \simeq –1.5 ) and high-N ( [ N/ \alpha ] \simeq –0.9 ) DLAs occurs at a nitrogen abundance of [ N/H ] \simeq - 2.8 , suggesting that the separation may result from some peculiarity of the nitrogen enrichment history .
The [ 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 can not exclude a primary origin in intermediate mass stars as responsible for the low N abundances observed .