Metal Abundances of Intermediate-redshift Active Galactic Nuclei: Evidence for a Population of Lower-metallicity Seyfert 2 Galaxies at z = 0.3–0.4

Abstract

We derive oxygen abundances for two samples of Seyfert 2 (Sy2) active galactic nuclei (AGNs) selected from the KPNO International Spectroscopic Survey (KISS). The two samples from KISS include 17 intermediate-redshift (0.29 ≤ z ≤ 0.42) Sy2s detected via their [O iii] lines, and 35 low-redshift (z ≤ 0.1), Hα-detected Sy2s. The primary goal of this work is to explore whether the metallicity distribution of these two samples changes with redshift. To determine the oxygen abundances of the KISS galaxies, we use Cloudy to create a large number of photoionization model grids by varying the temperature of the accretion disk, the ratio of X-ray to UV continuum light, the ionization parameter, the hydrogen density, and the metallicity of the narrow-line region clouds. We link the results of these models to the observed [O iii]/Hβ and [N ii]/Hα emission-line ratios of the KISS sample on a Baldwin–Philips–Terlevich diagram, interpolating across the model grids to derive metallicity. The two redshift samples overlap substantially in terms of derived metal abundances, but we find that some of the intermediate-redshift Sy2s possess lower abundances than their local universe counterparts. Our analysis provides evidence for modest levels of chemical evolution (0.18 ± 0.06 dex) over 3–4 Gyr of look-back time. We compare our results to other AGN abundance derivation methods from the literature.