Optical emission line diagnostics, which are a common tool to constrain the properties of the interstellar medium (ISM) of galaxies, become progressively inaccessible at higher redshifts for ground-based facilities. Far-infrared (FIR) emission lines, which are redshifted into atmospheric windows accessible by ground-based sub-millimeter facilities, could provide alternative ISM diagnostics to optical emission lines. We investigate FIR line ratios involving [CII]λ 158 μm, [OIII]λ 88 μm, [OIII]λ 52 μm, [NII]λ 122 μm and [NIIIλ 57 μm, using synthetic emission lines applied to a high-resolution (mgas= 883.4 M_) cosmological zoom-in simulation, including radiative-transfer post processing with KramsesRT at z = 6.5. We find that the [CII]/[NII]122 ratio is sensitive to the temperature and density of photo-dissociation regions, and thus could be a useful tool to trace the properties of this gas phase in galaxies. We also find that [NII]/[NIII] is a good tracer of the temperature and [OIII]52/[OIII]88 a good tracer of the gas density of HII regions. Emission line ratios containing the [OIII]λ 88 μm line are sensitive to high velocity outflowing gas.
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Schimek et al. (2024) studied this question.
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