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

Abstract

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]$\lambda 158\mu$m, [OIII]$\lambda 88\mu$m, [OIII]$\lambda 52\mu$m, [NII]$\lambda 122\mu$m and [NIII$\lambda 57\mu$m, using synthetic emission lines applied to a high-resolution (m${}_{\mathrm{gas}}$= 883.4 M${}_{\odot }$) 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]$\lambda 88\mu$m line are sensitive to high velocity outflowing gas.

Accepted for publication to A&A. 7 pages, 5 figures

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