Unveiling the Ionized and Neutral ISM at z > 10 : The Origin of [O III] /[C II] Ratios from a Sub-parsec Resolution Radiative Transfer Simulation

Abstract: Recent multi-wavelength observations by JWST and ALMA are unveiling both ionized and neutral ISM components in high-redshift ($z>6$) galaxies. In this work, we investigate the origin of rest-frame far-infrared [OIII]88 $\mu$m and [CII]158 $\mu$m emission by performing zoom-in cosmological simulations of dwarf-galaxy progenitors at $z=9-13$. Our simulations incorporate on-the-fly radiative transfer at sub-pc ($\sim$ 0.1 pc) resolution, allowing us to resolve the multi-phase ISM. We compute emission lines on a cell-by-cell basis, taking into account local temperature, density, metallicity, radiation field strength, column density, and spectral hardness of radiation bins. We find that [OIII] predominantly arises from centrally located ionizing bubbles with temperatures of $\sim (1-5)\times 10^4\,\mathrm{K}$ and high ionization parameters of $\log U_{\mathrm{ion}} \simeq -1.5$. In contrast, [CII] is produced in the surrounding dense neutral regions at $\sim 5\times 10^3\,\mathrm{K}$, which are heated by strong FUV radiation from the central stellar clusters. This spatial arrangement leads to large local variations in [OIII]/[CII], ranging from $\sim$ 100 to 0.01. Our galaxy reproduces the global ratio [OIII]/[CII]$\sim5-30$, consistent with recent ALMA detections at $z>6$ without invoking enhanced O/C abundance ratios. We further derive that [OIII]/[CII] linearly scales with the mass and density ratios of ionized to neutral gas, $M_{\rm HII}/M_{\rm HI}$ and $n_{\rm HII}/n_{\rm HI}$ and show that the [OIII]/[CII] ratio typically changes from 5.7 to 0.3 from high-z to low-z. For future synergies of JWST and ALMA, we derived $M_{\rm HII}/M_{\rm HI}$ for observed $z >6$ galaxies using ${\rm H}\beta$ and [CII] and show the validity of our scaling relations.