The ESO UVES Advanced Data Products Quasar Sample - III. Evidence of Bimodality in the [N/alpha] Distribution

TL;DR

This study analyzes nitrogen and alpha-element abundances in 108 Damped Lyα Absorbers, revealing a bimodal distribution of [N/α] ratios that suggests different nitrogen production mechanisms in early galaxies.

Contribution

It provides the first extensive evidence of bimodality in [N/α] ratios in DLAs, linking it to primary nitrogen production by different stellar populations.

Findings

Confirmation of bimodal [N/α] distribution in DLAs.

Identification of a lower [N/α] limit around -1.7.

Implication of primary nitrogen production by fast-rotating massive stars.

Abstract

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.