Chemical abundances in Seyfert galaxies -- VIII. Argon abundance estimates

TL;DR

This study measures argon abundances in Seyfert 2 galaxy nuclei using optical emission lines and photoionization models, revealing a range of argon levels and a slight decrease in Ar/O ratio with increasing oxygen abundance.

Contribution

First direct determination of Ar/H abundance in Seyfert 2 nuclei using the Te-method and photoionization models, expanding understanding of AGN chemical compositions.

Findings

Argon abundance ranges from 0.1 to 3 times solar in Seyfert 2 nuclei.

Ar/O ratio decreases slightly as O/H increases.

Results align with abundance gradients in spiral galaxy centers.

Abstract

For the first time, the argon abundance relative to hydrogen abundance (Ar/H) in the narrow line region of a sample of Seyfert 2 nuclei has been derived. In view of this, optical narrow emission line intensities of a sample of 64 local Seyfert 2 nuclei (z < 0.25) taken from Sloan Digital Sky Survey DR7 and measured by the MPA/JHU group were considered. We adopted the Te-method for AGNs, which is based on direct determination of the electron temperature, together with a grid of photoionization model results, built with the Cloudy code, to obtain a method for the derivation of the Ar/H abundance. We find that for a metallicity range of 0.2 < (Z/Zsolar) < 2.0, Seyfert 2 nuclei present Ar/H abundance ranging from 0.1 to 3 times the argon solar value, adopting log(O/H)=-3.31 and log(Ar/H)=-5.60. These range of values correspond to 8.0 < 12+log(O/H)< 9.0 and 5.4 < 12+log(Ar/H)< 6.9, respectively. The range of Ar/H and Ar/O abundance values obtained from our sample are in consonance with estimations from extrapolations of the radial abundance gradients to the central parts of the disk for four spiral galaxies. We combined our abundance results with estimates obtained from a sample of Hii galaxies, which were taken from the literature, and found that the Ar/O abundance ratio decreases slightly as the O/H abundance increases.