Compact Ionized Gas Region Surrounded by Porous Neutral Gas in a Dusty Lyman Break Galaxy at Redshift <i>z</i> = 8.312

Abstract 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 μ m/[C ii ] 158 μ m ratios found in UV-luminous star-forming galaxies at z &gt; 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 μ m / L [C II ] 158 μ m ratio (≈9) and dust continuum detection. We construct a multiphase ISM model incorporating the neutral gas covering fraction ( c ov 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 μ m/[C ii ] 158 μ m 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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>n</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> <mml:mo>,</mml:mo> <mml:mi mathvariant="normal">c</mml:mi> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">cm</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> <mml:mo>=</mml:mo> <mml:mn>2.7</mml:mn> </mml:math> ) 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.