Evidence for Changing Metal Abundances with Redshift of KISS AGN

We derive oxygen abundances for two samples of Seyfert 2 (Sy2) active galactic nuclei (AGN) 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 53 low redshift (z ≤ 0.1), Hα detected Sy2s. A primary goal of this project is to determine if the metallicity of these two samples changes with redshift. To determine the oxygen abundances of our sample of 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, and the hydrogen density and 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 our sample on the BPT diagram, interpolating across the model grid to derive metallicity. The two redshift samples overlap substantially in terms of derived metal abundances, but we find that some of the intermediate redshift Sy2 galaxies possess lower abundances than their local universe counterparts. This provides evidence for modest levels of chemical evolution over 3-4 Gyrs of look-back time.