Emission-line Diagnostics of Low Metallicity AGN

TL;DR

This study investigates whether AGN can reside in low-metallicity environments by combining photoionization modeling of narrow-line regions with SDSS spectroscopy. It finds that nitrogen-based line ratios are robust metallicity indicators for AGN, while other lines suffer from density-structure uncertainties. The SDSS analysis reveals a clear mass–metallicity trend in AGN hosts, with most AGN exhibiting solar or supersolar metallicities even at low host masses, and only a small, rare subset showing sub-solar NLR metallicities. The results indicate that truly low-metallicity AGN are uncommon in the local universe, with implications for interpreting high-redshift AGN and SMBH–galaxy co-evolution; future work should extend diagnostics to higher redshift and fainter AGN populations.

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 [NII] are the most robust metallicity indicators in galaxies where the primary source of ionization is from the active nucleus. Ratios involving [SII] and [OI] 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 [NII] 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-fit models for typical Seyfert narrow line regions 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 sub-solar metallicities are very rare in the SDSS. Out of a sample of ~23000 Seyfert 2 galaxies we find only ~40 clear candidates for AGN with NLR abundances that are below solar.

Figures (11)

• Figure 1: The BPT81 emission line ratio diagnostic diagram of [N ii]$\lambda 6584$Å/$\hbox{H}\alpha$ versus [O iii]$\lambda 5007$Å/$\hbox{H}\beta$ is plotted for SDSS emission line galaxies. Star-forming galaxies are shown in green, Seyfert 2s in blue and LINERs in red. The two different curves used to separate AGN and star bursts are indicated by the solid curve (Kauffmann separator) and dashed curve (Kewley separator).
• Figure 2: The BPT diagram of [N ii]/$\hbox{H}\alpha$ versus [O iii]/$\hbox{H}\beta$ demonstrating the decrease of metal emission lines relative to hydrogen. This decrease is stronger for the nitrogen lines due to the secondary component in this element. Model metallicities are as labelled, with the small numbers next to model indicating model run and associated ionization parameter as given in table \ref{['tab:U']}. The background image shows the log scale density distribution of the SDSS emission line sample, with Seyfert galaxies lying in the right, blue colored branch. Each branch has been scaled separately to emphasize the position of the Seyfert galaxies.
• Figure 3: Metallicity diagnostic diagram of [N ii]$\lambda 6584$Å/[O ii]$\lambda 3727$Å versus [O iii]$\lambda 5007$Å/ [O ii]$\lambda 3727$Å with models increasing in metallicity from left to right (as labelled). The background image shows the distribution of the SDSS emission line galaxies, with Seyfert galaxies, colored blue, lying in the vertical branch and starbursts in the horizontal branch. Each branch has been scaled separately to emphasize the position of the Seyfert galaxies. The contribution by star formation to the AGN decreases the [O iii]/[O ii] ratio.
• Figure 4: Near--UV metallicity diagnostic diagram of [O ii]$\lambda3727,9$Å/[Ne iii]$\lambda3869$Å versus [Ne v]$\lambda3426$Å/[Ne iii]$\lambda3869$Å. Pure star forming galaxies would lie below these curves at low [Ne v]/[Ne iii] values.
• Figure 5: The Stellar Mass distribution of the full emission line galaxy sample (solid line) and the AGN (including LINER) dominated galaxies (dashed line) in Solar masses. The peaks have been scaled by 11013 and 2286 for the full and AGN sample respectively.