Negative gas-phase metallicity gradients in the narrow-line region and galactic disc of local galaxies hosting active galactic nuclei

¹ Department of Physics, University of Arkansas, 226 Physics Building, 825 West Dickson Street, Fayetteville, AR 72701, USA
² Instituto de Astrofísica de Canarias, E-38205 La Laguna, Tenerife, Spain
³ Departamento de Astrofísica, Universidad de La Laguna, E-38200 La Laguna, Tenerife, Spain
⁴ Department of Physics & Astronomy “Augusto Righi”, University of Bologna, via Gobetti 93/2, 40129 Bologna, Italy
⁵ INAF, Astrophysics and Space Science Observatory Bologna, Via P. Gobetti 93/3, I-40129 Bologna, Italy

^⋆ Corresponding author: amirnezamamiri@gmail.com

Received: 4 August 2025
Accepted: 11 September 2025

Abstract

The gas-phase metallicity distribution in galaxies offers significant information on their evolution. We report the discovery of negative radial gradients in the gas-phase metallicity of the narrow-line region (NLR) of the nine galaxies in the Measuring Active Galactic Nuclei Under MUSE Microscope (MAGNUM) galaxies: Centaurus A, Circinus, IC 5063, NGC 1068, NGC 1365, NGC 1386, NGC 2992, NGC 4945, and NGC 5643. From the strong-line abundance relations for active galactic nuclei (AGNs) and star-forming regions, along with emission-line ratio diagnostics, we were able to determine spatially resolved gas-phase metallicities for the kinematic components, galaxy disc, and outflow. These relations involve sensitive strong emission lines, specifically [O III]λ5007, [N II]λ6584, Hα, Hβ, [S II]λ6716, and [S II]λ6731. The existence of predominantly negative radial metallicity gradients in these AGN host galaxies indicates that metals are not necessarily moved from the central regions to the outskirts by AGN activity. We find that the gas-phase metallicity in galaxies could follow the general inside-out star formation scenario.