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Tracing Molecular Gas Mass in z ≃ 6 Galaxies with [C ii]

We investigate the fine-structure [C ii] line at 158 μm as a molecular gas tracer by analyzing the relationship between molecular gas mass (M mol) and [C ii] line luminosity (L [C II]) in 11,125 z ≃ 6 star-forming, main-sequence galaxies from the simba simulations, with line emission modeled by the Simulator of Galaxy Millimeter/Submillimeter Emission. Though most (∼50%–100%) of the gas mass in our simulations is ionized, the bulk (>50%) of the [C ii] emission comes from the molecular phase. We find a sublinear (slope 0.78 ± 0.01) logL[CII]–logMmol relation, in contrast with the linear relation derived from observational samples of more massive, metal-rich galaxies at z ≲ 6. We derive a median [C ii]-to-M mol conversion factor of α [C II] ≃ 18 M ⊙/L ⊙. This is lower than the average value of ≃30 M ⊙/L ⊙ derived from observations, which we attribute to lower gas-phase metallicities in our simulations. Thus, a lower, luminosity-dependent conversion factor must be applied when inferring molecular gas masses from [C ii] observations of low-mass galaxies. For our simulations, [C ii] is a better tracer of the molecular gas than CO J = 1–0, especially at the lowest metallicities, where much of the gas is CO-dark. We find that L [C II] is more tightly correlated with M mol than with star formation rate (SFR), and both the logL[CII]–logMmol and logL[CII]–logSFR relations arise from the Kennicutt–Schmidt relation. Our findings suggest that L [C II] is a promising tracer of the molecular gas at the earliest cosmic epochs.