2024MNRAS.534.3040D

We derive the nitrogen and oxygen abundances in the narrow line regions (NLRs) of a sample of 38 local ($z \: \lt \: 0.4$) Seyfert 2 nuclei. For that, we consider narrow optical emission line intensities and direct estimates of the electron temperatures ($T_{\rm e}$-method). We obtain a new theoretical expression for the nitrogen ionization correction factor [ICF($\rm N^{+}$)] 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 $7.6 \: \lt \: \rm 12+log(N/H) \: \lt \: 8.6$ (mean value $8.06\pm 0.22$) or $\rm 0.4 \: \lt \: (N/N_{\odot }) \: \lt 4.7$, in the metallicity regime $8.3 \: \lt \: \rm 12+log(O/H) \: \lt \: 9.0$. Our results indicate that the dispersion in N/H abundance for a fixed O/H value in AGNs of $\sim 0.2$ 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 ($z \: \gt \: 5$) 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.