A theoretical investigation of far-infrared fine structure lines at $z>6$ and of the origin of the [OIII]88/[CII]158 enhancement

TL;DR

This study investigates why galaxies at z>6 exhibit unusually high [OIII] 88μm to [CII] 158μm luminosity ratios by applying Cloudy line modelling to a high-resolution Ponos zoom-in galaxy at z=6.5, incorporating non-Solar C/O and N/O abundance patterns inspired by JWST and local analogs. The results show that a substantially lower C/O ratio increases the [OIII]/[CII] ratio by about a factor of 4.5 and brings the total line luminosities into closer agreement with observations, while the [OIII]/[NII] ratio is highly sensitive to the assumed N/O pattern. The work highlights the crucial role of ISM chemistry and ionisation state in shaping FIR line emission at high redshift and provides guidance for interpreting ALMA and JWST data, as well as planning for future AtLAST surveys. It also demonstrates non-linearities and coupling between chemistry, gas structure, and radiative transfer that require further high-resolution, chemically detailed modelling to fully explain the observed diversity of high-z FIR line ratios.

Abstract

[Abridged] The [OIII]$_{88μm}$/[CII]$_{158μm}$ and [OIII]$_{88μm}$/[NII]$_{122μm}$ luminosity ratios have shown to be promising tracers of the ionisation state and gas-phase metallicity of the ISM. Observations of galaxies at redshift $z > 6$ show peculiarly higher [OIII]$_{88μm}$/[CII]$_{158μm}$ luminosity ratios compared to local sources. No model has so far successfully managed to match the observed emission from both [OIII]$_{88μm}$ and [CII]$_{158μm}$ as well as their ratio. We use Cloudy to model the [CII]$_{158μm}$, [OIII]$_{88μm}$, [NII]$_{122μm}$ and [NIII]$_{57μm}$ emission lines of Ponos: a high-resolution ($m_{\mathrm{gas}} = 883.4\, M_{\odot}$) cosmological zoom-in simulation of a galaxy at redshift $z = 6.5$, which is post-processed using kramses-rt. We modify Carbon, Nitrogen and Oxygen abundances in our Cloudy models to obtain C/O and N/O abundance ratios respectively lower and higher than Solar, more in line with recent high-z observational constraints. We find [OIII]$_{88μm}$/[CII]$_{158μm}$ luminosity ratios that are a factor of $\sim 5$ higher compared to models assuming solar abundances. Additionally, we find an overall better agreement of the simulation with high-z observational constraints of the [CII]$_{158μm}$-SFR and [OIII]$_{88μm}$-SFR relations. This shows that a lower C/O abundance ratio is essential to reproduce the enhanced [OIII]$_{88μm}$/[CII]$_{158μm}$ luminosity ratios observed at $z > 6$. By assuming a super-solar N/O ratio, motivated by recent $z > 6$ JWST observations, our models yield an [OIII]$_{88μm}$/[NII]$_{122μm}$ ratio of $1.3$, which, according to current theoretical models, would be more appropriate for a galaxy with a lower ionisation parameter than the one we estimated for Ponos.

Figures (14)

• Figure 1: Gas surface density map of the Ponos simulation (left panel). The dashed circle corresponds to the virial radius, $R_{\mathrm{vir}}$. The right panel shows the $\mathrm{[CII]}$ line surface density map obtained using the fiducial model of Schimek23Schimek24, which assumes solar elemental abundances, zoomed into the galaxy disc and the merging companions.
• Figure 2: $\mathrm{[CII]}$ line emission maps obtained with the Solar model (top panels) and the High-z model (bottom panels). The right panels are zoomed-in images of the tidal tail between merger B and merger A. The white dashed circles mark the extent of $R_{\mathrm{vir}}$ of Ponos. The line emission maps obtained with the Dwarfs and Halo models are shown in Appendix \ref{['app:dwarfs']}.
• Figure 3: $\mathrm{[OIII]}$ line emission maps obtained with the Solar model (top panels) and the High-z model (bottom panels). The right panels are zoomed-in images of the tidal tail between merger B and merger A. The white dashed circles mark the extent of $R_{\mathrm{vir}}$ of Ponos. The line emission maps obtained with the Dwarfs and Halo models are shown in Appendix \ref{['app:dwarfs']}.
• Figure 4: $\mathrm{[NII]}$ line emission maps obtained with the Solar model (top panels), the High-z model (middle panels), and the Dwarfs model (bottom panels). The right panels are zoomed-in images of the main disc and surrounding area. The white dashed circles mark the extent of $R_{\mathrm{vir}}$ of Ponos.
• Figure 5: Surface brightness ratio maps of the $\mathrm{[OIII]/[CII]}$ luminosity ratio, comparing the Solar model (left panel) and the High-z model (right panel). The title above each panel gives the total luminosity ratios. The ratio maps obtained with the Dwarfs and Halo models are shown in Appendix \ref{['app:dwarfs']}.