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