H II region filling factors in NGC 628: Luminosity-size relation and connection with polycyclic aromatic hydrocarbon emission
Abstract
Understanding the internal structure of H II regions is fundamental for constraining star formation processes in galaxies. We investigated how the filling factor (FF) relates to luminosity, size, electron density, and Hα equivalent width (EW(Hα)) in H II regions, and explored its connection with polycyclic aromatic hydrocarbon (PAH)-to-dust emission as a possible tracer of evolutionary stages. We analyzed 622 H II regions in NGC~628, combining 475 regions from SIGNALS and 147 from PHANGS-MUSE. We derived their luminosities, emission-line fluxes, radii, electron densities, and FF, and used PHANGS-JWST/MIRI imaging to quantify the PAH-to-dust ratio RPAH from the 7.7, 11.3, and 21~μm bands. Higher FF and EW(Hα) values are found in luminous regions, whereas more extended regions with lower EW(Hα) exhibit lower FF. We show that the H II region radius definition significantly affects the LHα--R relation. Low-luminosity compact H II regions appear to mark a transition from cluster-powered regions to nebulae ionized by single massive stars, around log(LHα) ∼ 37 erg s−1. The PAH-to-dust ratio correlates with the volumetric Hα luminosity density, LHα/R3, with a transition around log(LHα/R3) ∼ 32 erg s−1, corresponding to log(FF) ≈ −4.4. Regions with lower FF exhibit higher RPAH, suggesting less efficient PAH processing in more porous structures. These results are consistent with an evolutionary scenario in which FF decreases with stellar cluster age as giant H II regions evolve toward fainter and more extended states. The volumetric Hα luminosity density reduces covariance between LHα and R induced by region-definition methods, enabling more consistent cross-catalog comparisons.
Versions
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BibTeX
@misc{riesco2026region,
title = {{H II region filling factors in NGC 628: Luminosity-size relation and connection with polycyclic aromatic hydrocarbon emission}},
author = {Lorena Aragüete Riesco and José M. Vílchez and Salvador Duarte Puertas and Laurie Rousseau-Nepton and Carmelle Robert and René Pierre Martin and Philippe Amram and Robert Kennicutt Jr. and Christophe Morisset and Ray Garner III},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.00902},
eprint = {2609.00902},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.00902},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - H II region filling factors in NGC 628: Luminosity-size relation and connection with polycyclic aromatic hydrocarbon emission
AU - Lorena Aragüete Riesco
AU - José M. Vílchez
AU - Salvador Duarte Puertas
AU - Laurie Rousseau-Nepton
AU - Carmelle Robert
AU - René Pierre Martin
AU - Philippe Amram
AU - Robert Kennicutt Jr.
AU - Christophe Morisset
AU - Ray Garner III
PY - 2026
DA - 2026/09/01
PB - arXiv
DO - 10.48550/arxiv.2609.00902
UR - https://arxiv.org/abs/2609.00902
AB - Understanding the internal structure of H II regions is fundamental for constraining star formation processes in galaxies. We investigated how the filling factor (FF) relates to luminosity, size, electron density, and Hα equivalent width (EW(Hα)) in H II regions, and explored its connection with polycyclic aromatic hydrocarbon (PAH)-to-dust emission as a possible tracer of evolutionary stages. We analyzed 622 H II regions in NGC~628, combining 475 regions from SIGNALS and 147 from PHANGS-MUSE. We derived their luminosities, emission-line fluxes, radii, electron densities, and FF, and used PHANGS-JWST/MIRI imaging to quantify the PAH-to-dust ratio R_PAH from the 7.7, 11.3, and 21~μm bands. Higher FF and EW(Hα) values are found in luminous regions, whereas more extended regions with lower EW(Hα) exhibit lower FF. We show that the H II region radius definition significantly affects the L_Hα--R relation. Low-luminosity compact H II regions appear to mark a transition from cluster-powered regions to nebulae ionized by single massive stars, around log(L_Hα) ∼ 37 erg s^−1. The PAH-to-dust ratio correlates with the volumetric Hα luminosity density, L_Hα/R^3, with a transition around log(L_Hα/R^3) ∼ 32 erg s^−1, corresponding to log(FF) ≈ −4.4. Regions with lower FF exhibit higher R_PAH, suggesting less efficient PAH processing in more porous structures. These results are consistent with an evolutionary scenario in which FF decreases with stellar cluster age as giant H II regions evolve toward fainter and more extended states. The volumetric Hα luminosity density reduces covariance between L_Hα and R induced by region-definition methods, enabling more consistent cross-catalog comparisons.
ER -