Open AccessEmission‐line diagnostics of low‐metallicity active galactic nuclei
Author(s) -
Groves Brent A.,
Heckman Timothy M.,
Kauffmann Guinevere
Publication year - 2006
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1111/j.1365-2966.2006.10812.x
Subject(s) - physics , astrophysics , metallicity , galaxy , active galactic nucleus , redshift , astronomy , photoionization , line (geometry) , sky , stellar mass , emission spectrum , star formation , spectral line , ionization , ion , geometry , mathematics , quantum mechanics
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.