2026ApJ...998...26N

Recent multiwavelength observations by JWST and the Atacama Large Millimeter/submillimeter Array (ALMA) are unveiling both ionized and neutral interstellar medium (ISM) components in high-redshift (z > 6) galaxies. In this work, we investigate the origin of rest-frame far-infrared [O III] 88 μm and [C II] 158 μm emission by performing zoomed-in cosmological simulations of dwarf-galaxy progenitors at z = 9-13. Our simulations incorporate on-the-fly radiative transfer at subparsec (∼ 0.1 pc) resolution, allowing us to resolve the multiphase 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 [O III] predominantly arises from centrally located ionizing bubbles with temperatures of ∼(1-5) × 10⁴ K and high-ionization parameters of ${\mathrm{log}}{U}_{{\rm{ion}}}\simeq -1.5$. In contrast, [C II] is produced in the surrounding dense neutral regions at ∼5 × 10³ K, which are heated by strong far-UV radiation (G/G₀ ∼ 10^3–5) from the central stellar clusters. This spatial arrangement leads to large local variations in [O III] /[C II], ranging from ∼100-0.01. Our galaxy reproduces the global ratio [O III] /[C II] ∼5-30, consistent with recent ALMA detections at z > 6 without invoking enhanced O/C abundance ratios. We further derive that [O III] /[C II] linearly scales with the mass and density ratios of ionized to neutral gas, M_H II_/M_H I_ and n_H II_/n_H I_ and show that the [O III] /[C II] 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_H II_/M_H I_ for observed z > 6 galaxies using Hβ and [C II] and show the validity of our scaling relations.