Stellar associations powering HII regions $\unicode{x2013}$ II. Escape fraction of ionizing photons
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| Format: | Preprint |
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2026
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| author | Scheuermann, Fabian Kreckel, Kathryn Teh, Jia Wei Belfiore, Francesco Groves, Brent Barnes, Ashley T. Boquien, Médéric Chevance, Mélanie Dale, Daniel A. Egorov, Oleg Glover, Simon C. O. Grasha, Kathryn Hannon, Stephen Klessen, Ralf S. Larson, Kirsten L. Lee, Janice C. Liang, Fu-Heng Lopez, Laura A. Méndez-Delgado, J. Eduardo Neumann, Justus Ostriker, Eve Pan, Hsi-An Ramambason, Lise Santoro, Francesco Schinnerer, Eva Thilker, David A. Tian, Qiushi Chris Úbeda, Leonardo Whitmore, Bradley C. Williams, Thomas G. |
| author_facet | Scheuermann, Fabian Kreckel, Kathryn Teh, Jia Wei Belfiore, Francesco Groves, Brent Barnes, Ashley T. Boquien, Médéric Chevance, Mélanie Dale, Daniel A. Egorov, Oleg Glover, Simon C. O. Grasha, Kathryn Hannon, Stephen Klessen, Ralf S. Larson, Kirsten L. Lee, Janice C. Liang, Fu-Heng Lopez, Laura A. Méndez-Delgado, J. Eduardo Neumann, Justus Ostriker, Eve Pan, Hsi-An Ramambason, Lise Santoro, Francesco Schinnerer, Eva Thilker, David A. Tian, Qiushi Chris Úbeda, Leonardo Whitmore, Bradley C. Williams, Thomas G. |
| contents | Newly formed stars have a profound impact on 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 further our understanding of this process. In a sample of 19 nearby galaxies, we match HII regions from PHANGS$\unicode{x2013}$MUSE to their ionizing stellar source from PHANGS$\unicode{x2013}$HST and measure the percentage of ionizing radiation that is leaking into the surrounding diffuse ionized gas (DIG). Based on a catalogue, where each HII region is powered by a single young and massive stellar association, we measure a photon escape fraction of $f_\mathrm{esc}=82^{+12}_{-24}$ per cent. Comparable results are obtained when different procedures are used to match the ionized gas to its source. All samples we study contain a substantial fraction of objects (up to 20 per cent), where the stellar source is not sufficient to produce the H$α$ flux observed from the nebula. Many of them are probably related to uncertain age estimates, but we also find numerous regions, where a significant fraction of the ionizing photon budget is contributed by stars that reside outside the boundaries of the HII region. This motivates the use of an alternative galaxy-wide approach, in which we include all HII regions and stellar sources, not just the ones that show a clear overlap. When summing up the ionization budget over entire galaxies, we measure slightly lower, but consistent values. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2602_22151 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Stellar associations powering HII regions $\unicode{x2013}$ II. Escape fraction of ionizing photons Scheuermann, Fabian Kreckel, Kathryn Teh, Jia Wei Belfiore, Francesco Groves, Brent Barnes, Ashley T. Boquien, Médéric Chevance, Mélanie Dale, Daniel A. Egorov, Oleg Glover, Simon C. O. Grasha, Kathryn Hannon, Stephen Klessen, Ralf S. Larson, Kirsten L. Lee, Janice C. Liang, Fu-Heng Lopez, Laura A. Méndez-Delgado, J. Eduardo Neumann, Justus Ostriker, Eve Pan, Hsi-An Ramambason, Lise Santoro, Francesco Schinnerer, Eva Thilker, David A. Tian, Qiushi Chris Úbeda, Leonardo Whitmore, Bradley C. Williams, Thomas G. Astrophysics of Galaxies Newly formed stars have a profound impact on 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 further our understanding of this process. In a sample of 19 nearby galaxies, we match HII regions from PHANGS$\unicode{x2013}$MUSE to their ionizing stellar source from PHANGS$\unicode{x2013}$HST and measure the percentage of ionizing radiation that is leaking into the surrounding diffuse ionized gas (DIG). Based on a catalogue, where each HII region is powered by a single young and massive stellar association, we measure a photon escape fraction of $f_\mathrm{esc}=82^{+12}_{-24}$ per cent. Comparable results are obtained when different procedures are used to match the ionized gas to its source. All samples we study contain a substantial fraction of objects (up to 20 per cent), where the stellar source is not sufficient to produce the H$α$ flux observed from the nebula. Many of them are probably related to uncertain age estimates, but we also find numerous regions, where a significant fraction of the ionizing photon budget is contributed by stars that reside outside the boundaries of the HII region. This motivates the use of an alternative galaxy-wide approach, in which we include all HII regions and stellar sources, not just the ones that show a clear overlap. When summing up the ionization budget over entire galaxies, we measure slightly lower, but consistent values. |
| title | Stellar associations powering HII regions $\unicode{x2013}$ II. Escape fraction of ionizing photons |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2602.22151 |