Predictions of the L[C ii]–SFR and [Cii] Luminosity Function at the Epoch of Reionization

We present the first predictions for the L_{[C ii]}–SFR relation and [Cii] luminosity function (LF) in the epoch of reionization (EOR) based on cosmological hydrodynamics simulations using the simba suite plus radiative transfer calculations via sígame. The sample consists of 11,137 galaxies covering halo mass log M_halo ∈ [9, 12.4] M_⊙, star formation rate SFR ∈ [0.01, 330] M_⊙ yr⁻¹, and metallicity 〈Z_gas〉_SFR ∈ [0.1, 1.9] Z_⊙. The simulated L_{[C ii]}–SFR relation is consistent with the range observed, but with a spread of ≃0.3 dex at the high end of SFR (>100 M_⊙ yr⁻¹) and ≃0.6 dex at the lower end, and there is tension between our predictions and the values of L_{[C ii]} above 10^8.5 L_⊙ observed in some galaxies reported in the literature. The scatter in the L_{[C ii]}–SFR relation is mostly driven by galaxy properties, such that at a given SFR galaxies with higher molecular mass and metallicity have higher L_{[C ii]}. The [Cii] LF predicted by simba is consistent with the upper limits placed by the only existing untargeted flux-limited [Cii] survey at the EOR and those predicted by semianalytic models. We compare our results with existing models and discuss the differences responsible for the discrepant slopes in the L_{[C ii]}–SFR relation.