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

Aug 21, 2026

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Derivation of the gas-phase metallicity traced by O/H abundance for the narrow-line region (NLR) of 108 Seyfert galaxies, including radial metallicity gradients along host-galaxy discs, and a matched control sample without active nuclei. Using SDSS-IV MaNGA observational data and strong emission-line calibrations from the literature, the NLR metallicity is compared with the extrapolated central value from HII-region oxygen gradients. About 80% show lower NLR metallicity by 0.16–0.30 dex, attributed to accretion of metal-poor gas feeding the SMBH. Inverse correlations are reported with electron density, [O III] 5007, Hα luminosity, AV, and stellar mass.

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Abstract

1 Introduction

MNRAS 513, 807–821 (2022) [https://doi.org/10.1093/mnras/stac771](https://doi.org/10.1093/mnras/stac771) Advance Access publication 2022 March 22 Gas-phase metallicity determinations in nearby AGNs with SDSS-IV MaNGA: evidence of metal-poor accretion Jana´ına C. do Nascimento,1,2‹ Oli L. Dors , 1‹ Thaisa Storchi-Bergmann,2,3 N´ıcolas D. Mallmann,2,3 Rogrio Riffel ,2,3 Gabriele S. Ilha ,2,4 Rogemar A. Riffel ,2,4 Sandro B. Rembold,2,4 Alice Deconto-Machado,2,4 Luiz N. da Costa2,5 and Mark Armah3,5 1 Universidade do Vale do Para´ıba. Av. Shishima Hifumi, 2911, CEP: 12244-000 S˜ao Jos´e dos Campos, SP, Brazil 2Departamento de F´ısica, CCNE, Universidade Federal de Santa Maria, 97105-900 Santa Maria, RS, Brazil 3Laborat´orio Interinstitucional de e-Astronomia – LIneA, Rua General Jos´e Cristino 77, Rio de Janeiro, RJ – 20921-400, Brazil 4Departamento deAstronomia, IF, Universidade Federal do Rio Grande do Sul, CP 15051, 91501-970 Porto Alegre, RS, Brazil 5 Observat´orio Nacional, Rua General Jos´e Cristino, 77, Rio de Janeiro, RJ 20921-400, Brazil Accepted 2022 March 14. Received 2022 March 10; in original form 2021 September 3 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 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 NLRs was compared to the value derived from the extrapolation of the radial oxygen abundance gradient, obtained from HII region estimates along the galaxy disc, to the central part of the host galaxies. We ﬁnd that, for most of the objects (∼ 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 (Ne ),[O III]λ5007, and H α luminosities, extinction coefﬁcient (AV ) of the NLRs, as well as the stellar mass (M∗ ) of the host galaxies. Evidence of an inverse correlation between the D and the parameters Ne , M∗ , and Av was found. Key words: galaxies: abundances–galaxies: active–galaxies: evolution. 1 INTRODUCTION Emission-line intensities present in the optical spectra of active galactic nuclei (AGNs) and star-forming regions (SFs) are essential to estimate the physical properties of the gas phase, such as metallicity, chemical abundances of heavy elements (e.g. O, N, S), electron temperature, and electron density of these objects. In particular, due to their high luminosity and prominent emission lines, AGNs play a key role in studies of the chemical evolution of galaxies along the Hubble time. Oxygen abundance is usually used to estimate gas-phase metallicity of SFs (e.g. Kennicutt, Bresolin & Garnett 2003; Hagele et al. 2008; Yates, Kauffmann & Guo 2012) and of AGNs (e.g. StorchiBergmann et al. 1998; Dors et al. 2015, 2020b; Revalski et al. 2018; Flury & Moran 2020; Dors 2021) . This is due to the oxygen is the third most abundant element after hydrogen and helium and it presents prominent optical emission lines ([O II]λ3726, λ3729,[O III]λ5007) of its most abundant ions (O+ , O2+) measured inmost part of the spectra with high signal-to-noise ratio. In fact, Skillman & Kennicutt (1993) and Dors et al. (2020b) found that the ion abundances with ionization stage higher O2+ do not exceed 􀀄 E-mail: jananasci02@gmail.com (JCN); [dorsjunior@gmail.com](dorsjunior@gmail.com) (OLD) ∼ 5 per cent and ∼ 20 per cent of the total O/H abundance in SFsand AGNs, respectively. The

FAQs

How is gas-phase metallicity for the NLR determined in this study?

Metallicity is traced by the O/H abundance of the narrow-line region. It is derived using SDSS-IV MaNGA observational data together with strong emission-line calibrations from the literature.

What comparison is used to evaluate NLR metallicity versus host-galaxy gradients?

The NLR metallicity is compared with the value obtained by extrapolating the radial oxygen abundance gradient from HII-region estimates along the galaxy disc to the central host-galaxy region.

What explanation is suggested for the typical NLR metallicity being lower?

For most objects, the NLR metallicity is lower than the extrapolated value by 0.16–0.30 dex. The study attributes this difference to accretion of metal-poor gas onto the AGN that feeds the central SMBH.

Aug 21, 2026

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