Stellar associations powering H II regions

II. Escape fraction of ionising photons

¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12–14, 69120 Heidelberg, Germany
² Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
³ European Southern Observatory (ESO), Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
⁴ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50157 Firenze, Italy
⁵ International Centre for Radio Astronomy Research, University of Western Australia, 7 Fairway, Crawley, 6009 WA, Australia
⁶ Astronomy Australia Ltd. (Perth Office), University of Western Australia, 7 Fairway, Crawley, 6009 WA, Australia
⁷ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, 06000 Nice, France
⁸ Cosmic Origins Of Life (COOL) Research DAO, coolresearch.io
⁹ Department of Physics & Astronomy, University of Wyoming, Laramie, WY 82071, USA
¹⁰ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
¹¹ Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹² Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany
¹³ AURA for the European Space Agency (ESA), Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁴ Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA
¹⁵ Department of Astronomy, The Ohio State University, Columbus, OH 43210, USA
¹⁶ Instituto de Astronomía, Universidad Nacional Autónoma de México, Ap. 70-264, 04510 CDMX, Mexico
¹⁷ Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08544, USA
¹⁸ Department of Physics, Tamkang University, No. 151, Yingzhuan Road, Tamsui Dist., New Taipei City 251301, Taiwan
¹⁹ Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218, USA
²⁰ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
²¹ UK ALMA Regional Centre Node, Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester M13 9PL, UK

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 11 August 2025
Accepted: 7 February 2026

Abstract

Newly formed stars profoundly affect their environment by depositing energy and momentum into the surrounding gas. However, only a fraction of the stellar feedback is retained in the cloud, and observational constraints are needed to improve our understanding of this process. In a sample of 19 nearby galaxies, we matched H II regions from PHANGS–MUSE to their ionizing stellar source from PHANGS–HST and measured the percentage of ionizing radiation that leaks into the surrounding diffuse ionised gas (DIG). Based on a catalogue in which each H II region is powered by a single young and massive stellar association, we measure a photon escape fraction of f_esc = 82₋₂₄⁺¹² %. We obtain comparable results when using different procedures to match the ionised gas to its source. All samples in our study contain a substantial fraction of objects (up to 20%), in which the stellar source is insufficient to produce the H α flux observed from the nebula. Many of these cases are probably related to uncertain age estimates, but we also find numerous regions for which a significant fraction of the ionising photon budget is contributed by stars that reside outside the boundaries of the H II region. This finding motivates the use of an alternative galaxy-wide approach in which we include all H II regions and stellar sources, not just those that show a clear overlap. When we sum the ionisation budget over entire galaxies, we measure slightly lower, but consistent values.