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

Abstract

<jats:title>ABSTRACT</jats:title> <jats:p>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 (Te-method) and those derived using photoionization models. In view of this, observational emission-line ratios in the optical range ($3000 \: \lt \: \lambda (\mathring{\rm A}) \: \lt 7000$) 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 (t2) and high (t3) ionization gas zones derived for H ii regions in AGN chemical abundance studies. Using a photoionization model grid, we derived a new expression for t2 as a function of t3 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 Te-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.</jats:p>

Chemical abundances in Seyfert galaxies – V. The discovery of shocked emission outside the AGN ionization axis

“…Figure 4 shows the e[NII] versus e[OIII] plot with the observed values for Mrk 607 and Mrk 348, which are compared to predictions of AGN photoionization models built with the C code (Ferland et al 2013) by Dors et al (2020) and to integrated measurements for Seyfert 2 galaxies. While the points for Mrk 607 are seen in the same region occupied by the model predictions and by the integrated spectra, the points for Mrk 348 are located in a very distinct position, towards higher temperatures.…”

“…Direct determination of abundances, known as the e -method which is based on the observational determination of the electron temperature ( e ), yields more reliable abundance estimations because it circumvents the aforementioned problems of photoionization models. Recently, Dors et al (2020) presented a new methodology for the e -method to be applied to NLRs of Seyfert 2 nuclei, introducing a new relation between temperatures of the low ( 2 ) and high ( 3 ) ionization gas zones derived from photoionization models. This method produces O/H abundances slightly lower (about 0.2 dex) than those derived from detailed photoionization models.…”

“…The filled squares and circles show the median e values. The red stars are the AGN compilation of direct estimates of temperatures fromDors et al (2017Dors et al ( , 2020 and the red curve corresponds to predictions of their AGN photoionization models.…”

Chemical abundance of the LINER galaxy UGC 4805 with SDSS-IV MaNGA

“…In addition, photoionization models assuming electron density variations along the radius have an almost negligible influence on predicted optical line ratios as demonstrated by Dors et al (2019). For a detailed discussion about the 𝑁 e influence on metallicity estimates in Seyfert 2 AGNs, see Dors et al (2020b).…”

Polycyclic aromatic hydrocarbons and the ionized gas

“…The photoionization model grids were built using version 17.02 of the CLOUDY code (Ferland et al 2017) in order to compare the observational emission-line intensity ratios for the galaxies in our sample with the photoionization model predictions for similar emission-line intensity ratios. The input parameters of the models are briefly described in the following, however, for more details to these parameters see Dors et al (2015Dors et al ( , 2017Dors et al ( , 2020 and (Carvalho et al 2020).…”