H II region filling factors in NGC 628: Luminosity-size relation and connection with polycyclic aromatic hydrocarbon emission

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 H α equivalent width (EW(H α)) 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 μ m bands. Higher FF and EW(H α) values are found in luminous regions, whereas more extended regions with lower EW(H α) 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 (L Hα) about 37 erg s-1. The PAH-to-dust ratio correlates with the volumetric H α luminosity density, L Hα/R cubed, with a transition around (L Hα/R 3) about 32 erg s-1, corresponding to (FF) about-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 H α luminosity density reduces covariance between L Hα and R induced by region-definition methods, enabling more consistent cross-catalog comparisons.