Compact Ionized Gas Region Surrounded by Porous Neutral Gas in a Dusty Lyman Break Galaxy at Redshift $z=8.312$

Abstract

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] $88\hat{I}¼\mathrm{m}$-to-[C II] $158\hat{I}¼\mathrm{m}$ 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, MACS0416_Y1 with a high $L_{[\mathrm{OIII}]88}/L_{[\mathrm{CII}]158}$ ratio ($\approx 9$) and dust continuum detection. We construct a multi-phase ISM model incorporating the neutral gas covering fraction ($co{v}_{\mathrm{PDR}}$). The best-fit model reveals a $co{v}_{\mathrm{PDR}}\approx 25\%$, indicating that $\approx 75\%$ 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] $88\hat{I}¼\mathrm{m}/$[C II] $158\hat{I}¼\mathrm{m}$ 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 ($D=0.90\mathrm{pc}$) and gas density ($\log n_{\mathrm{H},\mathrm{c}}/\mathrm{c}{\mathrm{m}}^{-3}=2.7$) 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.

16 pages, 7 figures, accepted for publication in The Astrophysical Journal