2025ApJ...990...29H

Porous interstellar medium (ISM) structure in galaxies at the epoch of reionization gives us a hint to understand what types of galaxies contribute to reionization. Although recent studies have pointed out the positive correlation between high ionizing photon escape fractions and high [O III] 88 μm/[C II] 158 μm ratios found in UV-luminous star-forming galaxies at z > 6 with the Atacama Large Millimeter/submillimeter Array, previous studies have paid little attention to the neutral gas porosity that allows ionizing photons to escape. Here we present a detailed analysis of a z = 8.312 Lyman break galaxy, MACS0416_Y1 with a high L_[O III] 88 μm_/L_[C II] 158 μm_ ratio (≈9) and dust continuum detection. We construct a multiphase ISM model incorporating the neutral gas covering fraction (cov_PDR). The best-fit model reveals a cov_PDR ≈ 25%, indicating that ≈75% of the ionized gas region is exposed to intercloud space. We confirm that our conclusions hold even when varying star formation history, stellar age, gas/stellar metallicity, and carbon-to-oxygen abundance ratio. This finding meets one of the necessary conditions for galaxies to have a nonzero escape fraction of ionizing photons and supports recent studies that galaxies with a high [O III] 88 μm/[C II] 158 μm ratio, such as MACS0416_Y1, could contribute to cosmic reionization. Furthermore, the modeled H II region with the best-fitting parameters has a typical size (D = 0.90 pc) and gas density (${\mathrm{log}}\,{n}_{{\rm{H}},{\rm{c}}}/{{\rm{cm}}}^{-3}=2.7$) that are comparable to local compact H II regions. This suggests that the H II regions in MACS0416_Y1 are in an early evolutionary stage.