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 (Tₑ-method). 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 is in agreement 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.