Emission-line diagnostics of low-metallicity active galactic nuclei

Abstract

Current emission-line-based estimates of the metallicity of active galactic nuclei (AGN) at both high and low redshifts indicate that AGN have predominantly solar-to-supersolar metallicities. This leads to the question: do low-metallicity AGN exist? In this paper, we use photoionization models to examine the effects of metallicity variations on the narrow emission-lines from an AGN. We explore a variety of emission-line diagnostics that are useful for identifying AGN with low-metallicity gas. We find that line ratios involving [N ii] are the most robust metallicity indicators in galaxies where the primary source of ionization is from the active nucleus. Ratios involving [S ii] and [O i] are strongly affected by uncertainties in modelling the density structure of the narrow-line clouds. To test our diagnostics, we turn to an analysis of AGN in the Sloan Digital Sky Survey (SDSS). We find a clear trend in the relative strength of [N ii] with the mass of the AGN-host galaxy. The metallicity of the ISM is known to be correlated with stellar mass in star-forming galaxies; our results indicate that a similar trend exists for AGN. We also find that the best-fitting models for typical Seyfert narrow-line regions (NLRs) have supersolar abundances. Although there is a mass-dependent range of a factor of 2–3 in the NLR metallicities of the AGN in our sample, AGN with subsolar metallicities are very rare in the SDSS. Out of a sample of ∼23 000 Seyfert 2 galaxies, we find only ∼40 clear candidates for AGN with NLR abundances that are below solar.