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

Abstract We investigate the fine-structure C ii line at 158 μ m 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) log L C II – log M mol 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 log L C II – log M mol and log L C II – log SFR 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.