A physically motivated “charge-exchange method” for measuring electron temperatures within H ii regions

¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12-14, 69120 Heidelberg, Germany
e-mail: kathryn.kreckel@uni-heidelberg.de
² INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50157 Firenze, Italy
³ International Centre for Radio Astronomy Research, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
⁴ Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Str 2, 69120 Heidelberg, Germany
⁵ Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Im Neuenheimer Feld 205, 69120 Heidelberg, Germany
⁶ Center for Astrophysics and Space Sciences, Department of Physics, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093, USA
⁷ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁸ Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071, USA
⁹ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
¹⁰ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

Received: 25 April 2022
Accepted: 21 July 2022

Abstract

Aims. Temperature uncertainties plague our understanding of abundance variations within the interstellar medium. Using the PHANGS-MUSE large program, we develop and apply a new technique to model the strong emission lines arising from H ii regions in 19 nearby spiral galaxies at ~50 pc resolution and infer electron temperatures for the nebulae.

Methods. Due to the charge-exchange coupling of the ionization fraction of the atomic oxygen to that of hydrogen, the emissivity of the observed [O i]λ6300/Hα line ratio can be modeled as a function of the gas phase oxygen abundance (O/H), ionization fraction (f_ion), and electron temperature (T_e). We measure O/H using a strong-line metallicity calibration and identify a correlation between f_ion and [S iii]λ9069/[S ii]λ6716,6730, tracing ionization parameter variations.

Results. We solve for T_e and test the method by reproducing direct measurements of T_e([N ii]λ5755) based on auroral line detections to within ~600 K. We apply this“charge-exchange method” of calculating T_e to 4129 H ii regions across 19 PHANGS-MUSE galaxies. We uncover radial temperature gradients, increased homogeneity on small scales, and azimuthal temperature variations in the disks that correspond to established abundance patterns. This new technique for measuring electron temperatures leverages the growing availability of optical integral field unit spectroscopic maps across galaxy samples, increasing the statistics available compared to direct auroral line detections.