Gas phase metallicity determinations in nearby AGNs with SDSS-IV MaNGA: evidence of metal poor accretion

TL;DR

This study measures the metallicity of AGN narrow line regions using SDSS-IV MaNGA data, finding most are metal-poor compared to host galaxy gradients, suggesting accretion of metal-poor gas feeds the central black holes.

Contribution

It provides the first large-scale comparison of NLR metallicity with galaxy disk gradients in nearby AGNs, revealing evidence of metal-poor gas accretion.

Findings

Most NLR metallicities are lower than extrapolated disk values.

Average metallicity difference ranges from 0.16 to 0.30 dex.

Inverse correlation between metallicity difference and electron density, stellar mass, and extinction.

Abstract

We derive the metallicity (traced by the O/H abundance) of the Narrow Line Region ( NLR) of 108 Seyfert galaxies as well as radial metallicity gradients along their galaxy disks and of these of a matched control sample of no active galaxies. In view of that, observational data from the SDSS-IV MaNGA survey and strong emission-line calibrations taken from the literature were considered. The metallicity obtained for the NLRs %each Active Galactic Nucleus (AGN) was compared to the value derived from the extrapolation of the radial oxygen abundance gradient, obtained from \ion{H}{ii} region estimates along the galaxy disk, to the central part of the host galaxies. We find that, for most of the objects ($\sim 80\,\%$), the NLR metallicity is lower than the extrapolated value, with the average difference ($<D>$) between these estimates ranging from 0.16 to 0.30 dex. We suggest that $<D>$ is due to the accretion of metal-poor gas to the AGN that feeds the nuclear supermassive black hole (SMBH), which is drawn from a reservoir molecular and/or neutral hydrogen around the SMBH. Additionally, we look for correlations between $D$ and the electron density ($N_{\rm e}$), [\ion{O}{iii}]$\lambda$5007 and H$\alpha$ luminosities, extinction coefficient ($A_{V})$ of the NLRs, as well as the stellar mass ($M_{*}$) of the host galaxies. Evidences of an inverse correlation between the $D$ and the parameters $N_{\rm e}$, $M_{*}$ and $A_{\rm v}$ were found.