2025/06/25 by Masato Hagimoto, Yoichi Tamura, Hagimoto, Masato +21
Physics and Astronomy · #Solar and Space Plasma Dynamics #Radio Astronomy Observations and Technology #Astrophysics and Cosmic Phenomena
paper · pdf · doi:10.48550/arxiv.2506.20734
Porous interstellar medium (ISM) structure in galaxies at the epoch of reionization (EoR) 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-to-[C II] 158~μm ratios found in UV-luminous star-forming galaxies at z > 6 with ALMA, 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, MACS0416Y1 with a high L_[OIII]88/L_[CII]158 ratio (≈9) and dust continuum detection. We construct a multi-phase ISM model incorporating the neutral gas covering fraction (covPDR). The best-fit model reveals a covPDR≈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 non-zero 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 MACS0416Y1, 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 (log nH,c/\mathrmcm-3=2.7) that are comparable to local compact H II regions. This suggests that the H II regions in MACS0416Y1 are in an early evolutionary stage.