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 e - method). We obtain a new theoretical expression for the nitrogen ionization correction factor [ICF(N + )] for NLRs. Applying this new ICF, we unexpectedly find that NLRs and disc H II re gions e xhibit similar ICF distributions. We find nitrogen abundances in the range 7 . 6 < 12 + log(N / H) < 8 . 6 (mean value 8 . 06 ±0 . 22) or 0 . 4 < (N / N ) < 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 of ∼ 0 . 2 dex agrees with that for disc 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.