The ESO UVES Advanced Data Products Quasar Sample - III. Evidence of
  Bimodality in the [N/alpha] 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 ESO-UVES Advanced Data Products (EUADP) database. We report 9 new
measurements, 5 upper and 4 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
$\langle$[N/$\alpha$]$\rangle=-0.85$ ($\pm$0.20 dex) and one-quarter ratios are
clustered at $\langle$[N/$\alpha$]$\rangle= -1.41$ ($\pm$0.14 dex). The high
[N/$\alpha$] plateau is consistent with the HII regions of dwarf irregular and
blue compact dwarf galaxies although extended to lower metallicities and could
be interpreted as the result of a primary nitrogen production by intermediate
mass stars. The low [N/$\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$]
$\approx$ $-1.7$. 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.
