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A theoretical investigation of far-infrared fine structure lines at z > 6 and of the origin of the [O  III ] 88 μm/[C  II ] 158 μmenhancement

Nyhagen, C. T. LU orcid ; Schimek, A. ; Cicone, C. ; Decataldo, D. and Shen, S. (2025) In Astronomy and Astrophysics 702.
Abstract

The far-infrared (FIR) fine structure lines [C II]158 μm, [O III]88 μm, [N II]122 μm, and [N III]57 μm are excellent tools for probing the physical conditions of the interstellar medium (ISM). The [O III]88 μm/[C II]158 μm and [O III]88 μm/[N II]122 μm 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 unusually high [O III]88 μm/[C II]158 μm luminosity ratios compared to local sources. The origin of the enhanced ratios has been investigated in the literature with different theoretical modelling approaches. However,... (More)

The far-infrared (FIR) fine structure lines [C II]158 μm, [O III]88 μm, [N II]122 μm, and [N III]57 μm are excellent tools for probing the physical conditions of the interstellar medium (ISM). The [O III]88 μm/[C II]158 μm and [O III]88 μm/[N II]122 μm 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 unusually high [O III]88 μm/[C II]158 μm luminosity ratios compared to local sources. The origin of the enhanced ratios has been investigated in the literature with different theoretical modelling approaches. However, no model has to date successfully managed to match the observed emission from both [O III]88 μm and [C II]158 μm, as well as their ratio. For this study we used CLOUDY to model the [C II]158 μm, [O III]88 μm, [N II]122 μm, and [N III]57 μm emission lines of PONOS, a high-resolution (m gas = 883.4 M ⊙) 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 [O III]88 μm/[C II]158 μm luminosity ratios that are a factor of ∼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 [C II]158 μm-SFR and [O III]88 μm-SFR relations. This shows that a lower C/O abundance ratio is essential to reproduce the enhanced [O III]88 μm/[C II]158 μm 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 [O III]88 μm/[N II]122 μm 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. Most current simulations adopt solar abundance patterns that are not adequate for recently observed high-z predictions. Our results showcase the importance of theoretical modelling efforts, coupled with high-resolution zoom-in simulations, and with parallel multi-tracer observations to understand the physical and chemical conditions of the ISM at z > 6.

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organization
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type
Contribution to journal
publication status
published
subject
keywords
galaxies: evolution, galaxies: high-redshift, galaxies: ISM, ISM: abundances, ISM: lines and bands, methods: numerical
in
Astronomy and Astrophysics
volume
702
article number
A260
publisher
EDP Sciences
external identifiers
  • scopus:105023824709
ISSN
0004-6361
DOI
10.1051/0004-6361/202452718
language
English
LU publication?
yes
id
58c9bf8f-dc09-4d72-9648-f708bfb5b138
date added to LUP
2026-01-13 13:29:32
date last changed
2026-01-27 12:43:23
@article{58c9bf8f-dc09-4d72-9648-f708bfb5b138,
  abstract     = {{<p>The far-infrared (FIR) fine structure lines [C II]<sub>158 μm</sub>, [O III]<sub>88 μm</sub>, [N II]<sub>122 μm</sub>, and [N III]<sub>57 μm</sub> are excellent tools for probing the physical conditions of the interstellar medium (ISM). The [O III]<sub>88 μm</sub>/[C II]<sub>158 μm</sub> and [O III]<sub>88 μm</sub>/[N II]<sub>122 μm</sub> 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 &gt; 6 show unusually high [O III]<sub>88 μm</sub>/[C II]<sub>158 μm</sub> luminosity ratios compared to local sources. The origin of the enhanced ratios has been investigated in the literature with different theoretical modelling approaches. However, no model has to date successfully managed to match the observed emission from both [O III]<sub>88 μm</sub> and [C II]<sub>158 μm</sub>, as well as their ratio. For this study we used CLOUDY to model the [C II]<sub>158 μm</sub>, [O III]<sub>88 μm</sub>, [N II]<sub>122 μm</sub>, and [N III]<sub>57 μm</sub> emission lines of PONOS, a high-resolution (m <sub>gas</sub> = 883.4 M <sub>⊙</sub>) 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 [O III]<sub>88 μm</sub>/[C II]<sub>158 μm</sub> luminosity ratios that are a factor of ∼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 [C II]<sub>158 μm</sub>-SFR and [O III]<sub>88 μm</sub>-SFR relations. This shows that a lower C/O abundance ratio is essential to reproduce the enhanced [O III]<sub>88 μm</sub>/[C II]<sub>158 μm</sub> luminosity ratios observed at z &gt; 6. By assuming a super-solar N/O ratio, motivated by recent z &gt; 6 JWST observations, our models yield an [O III]<sub>88 μm</sub>/[N II]<sub>122 μm</sub> 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. Most current simulations adopt solar abundance patterns that are not adequate for recently observed high-z predictions. Our results showcase the importance of theoretical modelling efforts, coupled with high-resolution zoom-in simulations, and with parallel multi-tracer observations to understand the physical and chemical conditions of the ISM at z &gt; 6.</p>}},
  author       = {{Nyhagen, C. T. and Schimek, A. and Cicone, C. and Decataldo, D. and Shen, S.}},
  issn         = {{0004-6361}},
  keywords     = {{galaxies: evolution; galaxies: high-redshift; galaxies: ISM; ISM: abundances; ISM: lines and bands; methods: numerical}},
  language     = {{eng}},
  publisher    = {{EDP Sciences}},
  series       = {{Astronomy and Astrophysics}},
  title        = {{A theoretical investigation of far-infrared fine structure lines at z > 6 and of the origin of the [O  III ] <sub>88 μm</sub>/[C  II ] <sub>158 μm</sub>enhancement}},
  url          = {{http://dx.doi.org/10.1051/0004-6361/202452718}},
  doi          = {{10.1051/0004-6361/202452718}},
  volume       = {{702}},
  year         = {{2025}},
}