Unveiling the Ionized and Neutral ISM at z > 10: The Origin of [O iii ] /[C ii ] Ratios from a Subparsec Resolution Radiative Transfer Simulation

Abstract 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 μ m and [C ii ] 158 μ m 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) × 10 4 K and high-ionization parameters of log U ion ≃ − 1.5 . In contrast, [C ii ] is produced in the surrounding dense neutral regions at ∼5 × 10 3 K, which are heated by strong far-UV radiation ( G / G 0 ∼ 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 H β and [C ii ] and show the validity of our scaling relations.