Direct estimates of nitrogen abundance for Seyfert 2 nuclei

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 regions exhibit 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-circle dot)<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 similar to 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.