Direct estimates of nitrogen abundance for Seyfert 2 nuclei

TL;DR

This study measures nitrogen and oxygen abundances in Seyfert 2 nuclei using optical emission lines and electron temperature estimates, revealing similarities with star-forming regions and implications for galaxy evolution.

Contribution

It provides direct nitrogen abundance estimates in Seyfert 2 nuclei and compares their N/O ratios with star-forming regions and high-redshift nitrogen-loud galaxies.

Findings

Nitrogen abundances range from 7.6 to 8.6 in 12+log(N/H)

Seyfert 2 nuclei follow a similar N/O-(O/H) relation as star-forming regions

High-redshift nitrogen-loud galaxies have N/O ratios consistent with local NLRs

Abstract

We derive the nitrogen and oxygen abundances in the Narrow Line Regions (NLRs) of a sample of 38 local ($z \: < \: 0.4$) Seyfert~2 nuclei. For that, we consider narrow optical emission line intensities and direct estimates of the electron temperatures ($T_{\rm e}$-method). We find nitrogen abundances in the range $7.6 \: < \: \rm 12+log(N/H) \: < \: 8.6$ (mean value $8.06\pm0.22$) or $\rm 0.4 \: < \: (N/N_{\odot}) \: < 4.7$, in the metallicity regime $8.3 \: < \: \rm 12+log(O/H) \: < \: 9.0$. Our results indicate that the dispersion in N/H abundance for a fixed O/H value in AGNs is in agreement with that for disc \ion{H}{ii} regions with similar metallicity. We show that Seyfert~2 nuclei follow a similar (N/O)-(O/H) relation to the one followed by star-forming objects. Finally, we find that active galaxies called as 'nitrogen-loud' observed at very high redshift ($z \: > \: 5$) show N/O values in consonance with those derived for local NLRs. This result indicates that the main star-formation event is completed in the early evolution stages of active galaxies.