DIGging into HII regions: the role of ionizing photon leakage and spectral hardening in coupling HII regions and diffuse ionized gas
Abstract
The optical emission-line ratios of star-forming galaxies are broadly understood in the framework of photoionization, yet some systematic trends remain unexplained. Using 50--100 pc resolution PHANGS-MUSE observations of 19 nearby galaxies, we investigate how escaping ionizing radiation couples HII regions to the diffuse ionized gas (DIG), analyzing thousands of HII regions and DIG sightlines. HII regions and the DIG form a continuous sequence across standard line-ratio diagnostic diagrams (e.g., BPT diagrams), where progressively fainter regions show elevated high- and low-ionization line ratios, and the DIG extends the same trends into the composite--LINER regime. Line-ratio variations with global specific star formation rate (sSFR) suggest progressively harder ionizing radiation with increasing star formation activity, contrary to expectations for emission increasingly dominated by older stellar populations. Line-of-sight mixing explains part of the behavior but cannot reproduce all line-ratio trends. We therefore develop a model in which ionizing photons escape HII regions and propagate through the diffuse ISM, undergoing geometric dilution, attenuation, and spectral hardening. The model predicts that progressively fainter HII regions are increasingly ionized by leaked radiation from neighboring luminous regions, implying radiative coupling between HII regions. Joint modeling of HII regions and the DIG reproduces the HII region luminosity distribution and DIG surface brightness for escape fractions of ∼0.4−0.6 and ionizing-photon mean free paths of ∼0.5−1 kpc, and qualitatively reproduces the observed line-ratio sequences and their dependence on sSFR. Our results suggest that ionizing photon leakage may provide an important non-local feedback channel linking compact star-forming sites to large-scale galaxy disks.
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BibTeX
@misc{baron2026digging,
title = {{DIGging into HII regions: the role of ionizing photon leakage and spectral hardening in coupling HII regions and diffuse ionized gas}},
author = {Dalya Baron and Kathryn Kreckel and Gwen C. Rudie and Karin M. Sandstrom and Alberto D. Bolatto and Guillermo A. Blanc and Oleg V. Egorov and Eva Schinnerer and Ashley T. Barnes and Enrico Congiu and Daniel A. Dale and Simon C. O. Glover and Jaeyeon Kim and Ralf S. Klessen and José Eduardo Méndez-Delgado and Justus Neumann and Ismael Pessa and Thomas G. Williams},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.07163},
eprint = {2610.07163},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.07163},
}
RIS
TY - UNPB
TI - DIGging into HII regions: the role of ionizing photon leakage and spectral hardening in coupling HII regions and diffuse ionized gas
AU - Dalya Baron
AU - Kathryn Kreckel
AU - Gwen C. Rudie
AU - Karin M. Sandstrom
AU - Alberto D. Bolatto
AU - Guillermo A. Blanc
AU - Oleg V. Egorov
AU - Eva Schinnerer
AU - Ashley T. Barnes
AU - Enrico Congiu
AU - Daniel A. Dale
AU - Simon C. O. Glover
AU - Jaeyeon Kim
AU - Ralf S. Klessen
AU - José Eduardo Méndez-Delgado
AU - Justus Neumann
AU - Ismael Pessa
AU - Thomas G. Williams
PY - 2026
DA - 2026/10/05
PB - arXiv
DO - 10.48550/arxiv.2610.07163
UR - https://arxiv.org/abs/2610.07163
AB - The optical emission-line ratios of star-forming galaxies are broadly understood in the framework of photoionization, yet some systematic trends remain unexplained. Using 50--100 pc resolution PHANGS-MUSE observations of 19 nearby galaxies, we investigate how escaping ionizing radiation couples HII regions to the diffuse ionized gas (DIG), analyzing thousands of HII regions and DIG sightlines. HII regions and the DIG form a continuous sequence across standard line-ratio diagnostic diagrams (e.g., BPT diagrams), where progressively fainter regions show elevated high- and low-ionization line ratios, and the DIG extends the same trends into the composite--LINER regime. Line-ratio variations with global specific star formation rate (sSFR) suggest progressively harder ionizing radiation with increasing star formation activity, contrary to expectations for emission increasingly dominated by older stellar populations. Line-of-sight mixing explains part of the behavior but cannot reproduce all line-ratio trends. We therefore develop a model in which ionizing photons escape HII regions and propagate through the diffuse ISM, undergoing geometric dilution, attenuation, and spectral hardening. The model predicts that progressively fainter HII regions are increasingly ionized by leaked radiation from neighboring luminous regions, implying radiative coupling between HII regions. Joint modeling of HII regions and the DIG reproduces the HII region luminosity distribution and DIG surface brightness for escape fractions of ∼0.4−0.6 and ionizing-photon mean free paths of ∼0.5−1 kpc, and qualitatively reproduces the observed line-ratio sequences and their dependence on sSFR. Our results suggest that ionizing photon leakage may provide an important non-local feedback channel linking compact star-forming sites to large-scale galaxy disks.
ER -