Constraining the physical properties of gas in high-z galaxies with far-infrared and submillimetre line ratios

Optical emission line diagnostics, which are a common tool for constraining 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 that are accessible for ground-based submillimetre facilities, could provide ISM diagnostics alternative to optical emission lines. We investigated FIR line ratios involving [CII] λ158 μm, [OIII] λ88 μm, [OIII] λ52 μm, [NII] λ122 μm, and [NIII] λ57 μm using synthetic emission lines applied to a high-resolution (m_gas = 883.4 M_⊙) cosmological zoom-in simulation, including radiative transfer post-processing with the code KRAMSES-RT at z = 6.5. We find that the [CII]/[NII]122 ratio is sensitive to the temperature and density of photodissociation regions. It might therefore be a useful tool for tracing the properties of this gas phase in galaxies. We also find that [NII]/[NIII] is a good tracer of the temperature and that [OIII]52/[OIII]88 is a good tracer of the gas density of HII regions. Emission line ratios containing the [OIII] λ88 μm line are sensitive to high-velocity outflowing gas.