The ESO UVES advanced data products quasar sample - III. Evidence of bimodality in the [N/α] distribution

We report here a study of nitrogen and alpha-capture element (O, S, and Si) abundances in 18 damped Ly alpha absorbers (DLAs) and sub-DLAs drawn from the European Southern Observatory's Ultraviolet Visual Echelle Spectrograph (ESO-UVES) Advanced Data Products (EUADP) data base. We report nine new measurements, five upper and four lower limits of nitrogen that when compiled with available nitrogen measurements from the literature makes a sample of 108 systems. The extended sample presented here confirms the [N/alpha] bimodal behaviour suggested in previous studies. Three-quarter of the systems show <[N/alpha]> = -0.85 (+/- 0.20 dex) and one-quarter of the systems show that ratios are clustered at <[N/alpha]> = -1.41 (+/- 0.14 dex). The high [N/alpha] plateau is consistent with the HII regions of dwarf irregular and blue compact dwarf galaxies although extended to lower metallicities and could be interpreted as the result of a primary nitrogen production by intermediatemass stars. The low [N/alpha] values are the lowest ever observed in any astrophysical site. In spite of this fact, even lower values could be measured with the present instrumentation, but we do not find them below [N/alpha] approximate to -1.7. This suggests the presence of a floor in [N/alpha] abundances, which along with the lockstep increase of N and Si may indicate a primary nitrogen production from fast rotating, massive stars in relatively young or unevolved systems.