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

We derive the metallicity (traced by the O/H abundance) of the narrow-line region (NIA) of 108 Seyfert galaxies as well as radial metallicity gradients along their galaxy discs 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 NLRB was compared to the value derived from the extrapolation of the radial oxygen abundance gradient, obtained from H it region estimates along the galaxy disc, to the central part of the host galaxies. We find that, for most of the objects (similar to 80 per cent), 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-e), [O III])lambda 5007, and H-alpha luminosities, extinction coefficient (A(V)) of the NLRB, as well as the stellar mass (M-*)( )of the host galaxies. Evidence of an inverse correlation between the D and the parameters N-e, M-*, and A(v) was found.