Chemical Abundances in Active Galaxies

TL;DR

This paper develops a new method to accurately determine chemical abundances in active galactic nuclei using SDSS data, improving upon previous techniques by incorporating ionization effects and strong emission line diagnostics.

Contribution

It introduces a novel, physically motivated approach to estimate AGN metallicities from emission line ratios, achieving high accuracy and enabling large-scale abundance measurements.

Findings

Strong correlation between electron temperature and oxygen abundance.

Nitrogen and oxygen abundances are strongly correlated in AGN.

Developed a single-equation method for abundance determination from BPT line ratios.

Abstract

The Sloan Digital Sky Survey (SDSS) has proved to be a powerful resource for understanding the physical properties and chemical composition of star-forming galaxies in the local universe. The SDSS population of active galactic nuclei (AGN) remains as of yet less explored in this capacity. To extend the rigorous study of \ion{H}{ii} regions in the SDSS to AGN, we adapt methods for computing direct-method chemical abundances for application to the narrow-line regions (NLR) of AGN. By accounting for triply-ionized oxygen, we are able to more completely estimate the total oxygen abundance. We find a strong correlation between electron temperature and oxygen abundance due to collisional cooling by metals. Furthermore, we find that nitrogen and oxygen abundances in AGN are strongly correlated. From the metal-temperature relation and the coupling of nitrogen and oxygen abundances, we develop a new, empirically and physically motivated method for determining chemical abundances from the strong emission lines commonly employed in flux-ratio diagnostic diagrams (BPT diagrams). Our approach, which for AGN reduces to a single equation based on the BPT line ratios, consistently recovers direct-method abundances over a 1.5 dex range in oxygen abundance with an rms uncertainty of 0.18 dex. We have determined metallicities for thousands of AGN in the SDSS, and in the process have discovered an ionization-related discriminator for Seyfert and LINER galaxies.