Temperature-based radial metallicity gradients in nearby galaxies

¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12-14, D-69120 Heidelberg, Germany
² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
³ Universidad Nacional Autónoma de México, Instituto de Astronomía, AP 70-264, CDMX 04510, México
⁴ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50157 Firenze, Italy
⁵ Department of Physics, Astronomy Section, University of Trieste, Via G.B. Tiepolo, 11, I-34143 Trieste, Italy
⁶ European Southern Observatory, Avenida Alonso de Cordoba 3107, Casilla 19, Santiago 19001, Chile
⁷ Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071, USA
⁸ Department of Astronomy, University of Cape Town, Rondebosch 7701, Cape Town, South Africa
⁹ Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Str 2, D-69120 Heidelberg, Germany
¹⁰ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
¹¹ Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Im Neuenheimer Feld 205, D-69120 Heidelberg, Germany
¹² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹³ Elizabeth S. and Richard M. Cashin Fellow at the Radcliffe Institute for Advanced Studies at Harvard University, 10 Garden Street, Cambridge, MA 02138, USA
¹⁴ Department of Physics, Tamkang University, No.151, Yingzhuan Rd., Tamsui Dist., New Taipei City 251301, Taiwan
¹⁵ Departamento de Física de la Tierra y Astrofísica, Universidad Complutense de Madrid, E-28040, Spain
¹⁶ Sub-department of Astrophysics, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK

^⋆ Corresponding author: kathryn.kreckel@uni-heidelberg.de

Received: 18 June 2025
Accepted: 28 July 2025

Abstract

Context. Gas-phase abundances provide insights into the baryon cycle, with radial gradients and 2D metallicity distributions tracking how metals are built up and redistributed across galaxy disks over cosmic time.

Aims. We use a catalog of 22 958 H II regions across 19 nearby spiral galaxies to examine how precisely the radial abundance gradients can be traced when using only the [N II] λ5755 electron temperature as a proxy for temperature-based, direct method metallicities.

Methods. Using 534 direct detections of the temperature sensitive [N II] λ5755 auroral line, we measured gradients in 15 of the galaxies. Leveraging our large catalog of individual H II regions, we carried out a stacking procedure in bins of the H II region [N II] λ6583 luminosity and radius to recover stacked radial gradients.

Results. We found a good agreement between the metallicity gradients from the stacked spectra and those gradients from individual regions and those from strong-line methods. In addition, particularly in the stacked T_e[N II] measurements, some galaxies show very low (< 0.05 dex) scatter in metallicities, indicative of a well-mixed ISM. We examined the individual high confidence (S/N > 5) outliers and identified 13 regions across nine galaxies with anomalously low metallicities, although this is not strongly reflected in the strong-line method metallicities. By stacking arm and interarm regions, we found no systematic evidence for offsets in metallicity between these environments, suggesting that enrichment within spiral arms is due to very localized processes.

Conclusions. This work demonstrates the potential to systematically exploit the single [N II] λ5755 auroral line for detailed gas-phase abundance studies of galaxies. It provides strong validation of previous results, based on the strong-line calibrations, of a well-mixed ISM across typical star-forming spiral galaxies.