SIMBAD references

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.