Chemical abundances of Seyfert 2 AGNs - III. Reducing the oxygen abundance discrepancy

Abstract

We investigate the discrepancy between oxygen abundance estimations for narrow-line regions of active galactic nuclei (AGNs) type Seyfert 2 derived using direct estimations of the electron temperature (T-e-method) and those derived using photoionization models. In view of this, observational emission-line ratios in the optical range (3000 < lambda(angstrom) < 7000) of Seyfert 2 nuclei compiled from the literature were reproduced by detailed photoionization models built with the CLOUDY code. We find that the derived discrepancies are mainly due to the inappropriate use of the relations between temperatures of the low (t(2)) and high (t(3)) ionization gas zones derived for H II regions in AGN chemical abundance studies. Using a photoionization model grid, we derived a new expression for t(2) as a function of t(3) valid for Seyfert 2 nuclei. The use of this new expression in the AGN estimation of the O/H abundances based on Te-method produces O/H abundances slightly lower (about 0.2 dex) than those derived from detailed photoionization models. We also find that the new formalism for the T-e-method reduces by about 0.4 dex the O/H discrepancies between the abundances obtained from strong emission-line calibrations and those derived from direct estimations. © 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society