Monthly Notices of the Royal Astronomical Society | 2021
Chemical abundances in Seyfert galaxies – VII. Direct abundance determination of neon based on optical and infrared emission lines
Abstract
\n For the first time, neon abundance has been derived in the narrow line region from a sample of Seyfert\xa02 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 ($z \\:\\lesssim \\:0.06$). The relative intensities of emission lines were used to derive the ionic and total neon and oxygen abundances through electron temperature estimations (Te-method). For the neon, abundance estimates were obtained by using both Te-method and IR-method. Based on photoionization model results, we found a lower electron temperature [$t_{\\rm e}({\\rm Ne\\, \\small {III}})$] for the gas phase where the Ne2 + is located in comparison with t3 for the O2 + ion. We find that the differences (D) between Ne2 +/H+ ionic abundances calculated from IR-method and Te-method (assuming t3 in the Ne2 +/H+ derivation) are similar to the derivations in star-forming regions (SFs) and they are reduced by a mean factor of ∼3 when $t_{\\rm e}({\\rm Ne\\, \\small {III}})$ is considered. We propose a semi-empirical Ionization Correction Factor (ICF) for the neon, based on [Ne\u2009ii]12.81$\\rm{\\mu m}$, [Ne\u2009iii]15.56$\\rm{\\mu m}$, and oxygen ionic abundance ratios. We find that the average Ne/H abundance for the Seyfert 2s sample is nearly 2\xa0times higher than similar estimate for SFs. Finally, for the very high metallicity regime (i.e. [$\\rm 12+log(O/H)\\: \\gtrsim \\: 8.80$]) an increase in Ne/O with O/H is found, which likely indicates secondary stellar production for the neon.