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

Abstract: [Abridged] The [OIII]${88\mu m}$/[CII]${158\mu m}$ and [OIII]${88\mu m}$/[NII]${122\mu m}$ 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]${88\mu m}$/[CII]${158\mu m}$ luminosity ratios compared to local sources. No model has so far successfully managed to match the observed emission from both [OIII]${88\mu m}$ and [CII]${158\mu m}$ as well as their ratio. We use Cloudy to model the [CII]${158\mu m}$, [OIII]${88\mu m}$, [NII]${122\mu m}$ and [NIII]${57\mu m}$ 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]${88\mu m}$/[CII]${158\mu m}$ 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]${158\mu m}$-SFR and [OIII]${88\mu m}$-SFR relations. This shows that a lower C/O abundance ratio is essential to reproduce the enhanced [OIII]${88\mu m}$/[CII]${158\mu m}$ 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]${88\mu m}$/[NII]${122\mu m}$ 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.

• The paper demonstrates that lowering the C/O ratio in simulations produces [OIII]/[CII] ratios approximately five times higher than those using solar abundances.
• It combines a high-resolution zoom-in cosmological simulation of galaxy Ponos with the Cloudy photoionisation code to model ISM fine structure lines.
• Findings suggest rapid oxygen enrichment from core-collapse supernovae drives high redshift FIR emission, though additional factors may still be at play.

A Theoretical Investigation of Far-Infrared Fine Structure Lines at High Redshifts

The study by Nyhagen et al. explores the far-infrared (FIR) fine structure lines, focusing on redshifts greater than 6, to better understand the high [OIII]/[CII] luminosity ratios observed in such early epochs of the universe. Through the use of a sophisticated high-resolution zoom-in cosmological simulation of a galaxy, termed Ponos, combined with the Cloudy photoionisation code, the authors investigate the elemental abundances in the interstellar medium (ISM) and their effects on FIR line emission.

The emphasis of the work is the [OIII]$_{88\mu m}$/[CII]$_{158\mu m}$ ratio, which has been found to be anomalously high in galaxies at redshifts exceeding 6, in contrast with local galaxies. Previous theoretical works have struggled to simultaneously reproduce both the [OIII] and [CII] lines as well as their ratios at such early cosmic times. This paper addresses that challenge by tweaking the elemental abundances of Carbon (C), Nitrogen (N), and Oxygen (O) in their simulation models, reflecting non-solar abundance ratios motivated by recent observational constraints. These changes brought the C/O abundance ratio below solar, aligned with data from high-z galaxies observed with JWST.

The numerical results are notable. The simulation yielded [OIII]/[CII] ratios approximately five times greater than those assuming solar abundances, indicating that a lower C/O abundance ratio is necessary to reflect the observed enhancements seen in galaxies at z > 6. This deduced low C/O abundance aligns with the hypothesis of rapid enrichment primarily from core-collapse supernovae in the early universe, which predominantly contribute oxygen but lead to lower carbon abundances due to the different formation timescales and stellar sources of these elements.

Additionally, the paper probes the [OIII]$_{88\mu m}$/[NII]$_{122\mu m}$ ratio, suggesting that it might ordinarily imply a galaxy with lower ionisation parameters than what was modeled for Ponos. This component of the research underscores the complex interdependencies between abundance ratios, ionisation state, and metallicity within the ISM, offering important insights into the conditions of galaxies at the epoch of reionization.

In comparative analysis with observed galaxies, the work shows that while the revised C/O ratios help in partially bridging the gap between simulations and observations, the predicted [OIII]/[CII] ratios remain lower than those empirically observed, suggesting additional contributing factors not yet captured by current models. These might include variations in the ionisation parameter or other unknowns of the primordial ISM conditions.

The theoretical implications of this study are profound, pushing forward our understanding of galaxy formation and evolution in the early universe. Practically, it highlights the need for even more refined models incorporating dynamic elements and chemical evolution on smaller scales, suggesting directions for future observational campaigns and theoretical modeling efforts. Such detailed modeling will be critically important for effectively utilizing the forthcoming high-sensitivity, wide-field observations from future telescopes like the Atacama Large Aperture Submillimeter Telescope (AtLAST), which promises to deliver unparalleled insights into the early universe galaxies.

In conclusion, Nyhagen et al.'s research illustrates the difficulties present in interpreting high redshift galaxy emission and the importance of adapting theoretical models to physical conditions that are not yet fully understood. Future developments in both instrumentation and theoretical frameworks will be critical to resolving these complex interstellar environments, deepening our insight into galaxy evolution at cosmic dawn.