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

TL;DR

The study uses sub-parsec, on-the-fly radiative transfer simulations of a z>9 dwarf-galaxy to resolve the multi-phase ISM and compute rest-frame FIR line emission. It finds that [OIII] 88 μm primarily originates in central ionized bubbles with high ionization parameters, while [CII] 158 μm comes from surrounding neutral PDR gas, producing large local [OIII]/[CII] variations and a global ratio of ~5–30 consistent with observations without requiring enhanced O/C abundances. A simple scaling relation shows [OIII]/[CII] tracks the ionized-to-neutral gas mass and density ratios, and the evolution from high-z to low-z is driven by cooling rates, gas masses, densities, and abundances. The work demonstrates fruitful JWST–ALMA synergies for diagnosing ISM conditions, and suggests that bursty star formation and high ionization in early galaxies can naturally yield the observed high line ratios. While focusing on a single low-mass system, the results provide physically grounded insights applicable to more massive galaxies and guide interpretation of forthcoming high-z observations.

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 $μ$m and [CII]158 $μ$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 ($G/G_0 \sim 10^{3-5}$) 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 ${\rm H}β$ and [CII] and show the validity of our scaling relations.

Figures (17)

• Figure 1: Histograms of various physical properties of simulated cells at $z=10.5$ (snapshot 208), which marks the largest SFR during the burst of star formation. Panels show (a) ionization fraction, (b) gas number density, (c) gas temperature, (d) gas-phase metallicity, (e) gas column density, and (f) FUV radiation field. For panels (b)--(f), the orange histograms correspond to PDR cells ($y_{\rm HII} < 0.8$), and the blue histograms correspond to H ii cells ($y_{\rm HII} \geq 0.8$). The histograms are weighted by gas mass. Panel (a) is normalized by the total gas mass within the virial radius, whereas in panels (b)-(f), the histograms for ionized and neutral gas are weighted by the total ionized and neutral gas mass, respectively.
• Figure 1: Same as Figure \ref{['fig:projection']} but sliced maps at $z = 10.45$. Only $\Sigma_*$ is a projected map.
• Figure 1: Time evolution of the ionizing photon production rate (left) and FUV luminosity (right) as a function of stellar age, calculated from the BPASS single-star SED model assuming $M_* = 10^6\,M_\odot$ and instantaneous star formation. We also show models with different metallicities: $Z = 5 \times 10^{-4} Z_\odot$ (red), $Z = 5 \times 10^{-3} Z_\odot$ (green), and $Z = 5 \times 10^{-2} Z_\odot$ (blue).
• Figure 1: The CMB effect to [Cii] 158$\rm \mu \mathrm{m}~$ luminosity at $z=10$ as a function of hydrogen nuclei density and temperature. The color scale indicates the reduction strength $1-\eta$, where $\eta$ is the luminosity ratio of [Cii] with (without) the CMB stimulated emission and absorption. The blue solid line represents the critical density of hydrogen atoms. The gray contours show the [Cii] luminosity in the phase-diagram with $\log L_{\rm [CII]}/[L_\odot/\Delta\log n_{\rm H}/\Delta \log T])=-0.5, 0.5, 1.5$, where $\Delta\log n_{\rm H}=0.12$ and $\Delta \log T=0.05$. Note that the contoured [Cii] luminosities are calculated by considering the CMB effect in CLOUDY.
• Figure 2: Histograms of the radiation field at $z=10.5$ (snapshot 204). (a) The flux ratio of FUV to EUV1. Orange and blue histograms correspond to PDR and H ii cells, respectively. We find that the FUV flux in PDR cells exceeds the EUV flux by $\sim 20$ orders of magnitude, making the EUV component negligible. (b) The flux ratio of EUV1 to EUV2 for H ii cells. As in Figure \ref{['fig:parameter_histgram']}, the distributions are weighted by gas mass and normalized by the total ionized and neutral gas mass, respectively.