Direct estimates of nitrogen abundance for Seyfert 2 nuclei

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 ($T_{\mathrm{e}}$-method). We find nitrogen abundances in the range $7.6<12+\log (N/H)<8.6$ (mean value $8.06\pm 0.22$) or $0.4<(N/{\mathrm{N}}_{\odot })<4.7$, in the metallicity regime $8.3<12+\log (O/H)<9.0$. 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.

Submitted to MNRAS. Comments are welcome!