Chemical abundances in Seyfert galaxies -- VII. Direct abundance determination of neon based on optical and infrared emission lines

TL;DR

This study derives neon abundances in Seyfert 2 nuclei using optical and infrared emission lines, revealing higher neon levels compared to star-forming regions and suggesting secondary stellar production at high metallicities.

Contribution

First direct neon abundance measurements in Seyfert 2 nuclei using combined optical and IR emission lines, with a new semi-empirical ionization correction factor and insights into neon production.

Findings

Neon abundance in Seyfert 2s is nearly twice that in star-forming regions.

A lower electron temperature for Ne$^{2+}$ compared to O$^{2+}$ was found.

Ne/O ratio increases with metallicity, indicating secondary stellar production.

Abstract

For the first time, neon abundance has been derived in the narrow line region from a sample of Seyfert~2 nuclei. In view of this, we compiled from the literature fluxes of optical and infrared (IR) narrow emission lines for 35 Seyfert 2 nuclei in the local universe ($z < 0.06$). The relative intensities of emission lines were used to derive the ionic and total neon and oxygen abundances through electron temperature estimations ($T_{e}$-method). For the neon, abundance estimates were obtained by using both $T_{e}$-method and IR-method. Based on photoionization model results, we found a lower electron temperature [$t_{e}([Ne III])$] for the gas phase where the Ne$^{2+}$ is located in comparison with $t_{3}$ for the O$^{2+}$ ion. We find that the differences (D) between Ne$^{2+}$/H$^{+}$ ionic abundances calculated from IR-method and $T_{e}-$method (assuming $t_{3}$ in the Ne$^{2+}$/H$^{+}$ derivation) are similar to the derivations in star-forming regions (SFs) and they are reduced by a mean factor of $\sim3$ when $t_{e}([Ne III])$ is considered. We propose a semi-empirical Ionization Correction Factor (ICF) for the neon, based on [Ne II]12.81$\mu$m, [\ion{Ne}{iii}]15.56$\mu$m and oxygen ionic abundance ratios. We find that the average Ne/H abundance for the Seyfert 2s sample is nearly 2 times higher than similar estimate for SFs. Finally, for the very high metallicity regime (i.e. [$12+log(O/H) > 8.80$]) an increase in Ne/O with O/H is found, which likely indicates secondary stellar production for the neon.