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

We investigate the fine-structure [CII] line at 158μm as a molecular gas tracer by analyzing the relationship between molecular gas mass (Mmol) and [CII] line luminosity (L[CII]) in 11,125 z≃6 star-forming, main sequence galaxies from the SIMBA simulations, with line emission modeled by SÍGAME. Though most (∼50−100%) of the gas mass in our simulations is ionized, the bulk (>50%) of the [CII] emission comes from the molecular phase. We find a sub-linear (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 [CII]-to-Mmol conversion factor of α[CII]≃18M⊙/L⊙. This is lower than the average value of ≃30M⊙/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 [CII] observations of low-mass galaxies. For our simulations, [CII] 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[CII] is more tightly correlated with Mmol 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[CII] is a promising tracer of the molecular gas at the earliest cosmic epochs.