Left: [C II]/FIR luminosity ratio as a function of the FIR luminosity observed in nearby star-forming galaxies and starbursts (beige points; Lutz et al. 2016; Herrera-Camus et al. 2018b) and high-z star-forming galaxies (gray points; Spilker et al. 2016, and references therein). CRISTAL galaxies are shown as gray points with green borders. As a reference, the y axis on the right shows the [C II]/FIR luminosity ratio normalized to the value measured in the dense PDR of the Orion Nebula (L_[CII]/L_FIR = 1.1 × 10⁻³; Goicoechea et al. 2015). The predictions from the models by Narayanan & Krumholz (2017) of how the relation between L_[CII]/L_FIR and L_FIR changes if the molecular gas content increase by a factor of ×10 are shown as light-blue lines. The solid golden line represents the running median for z ≈ 0 galaxies, while the dashed lines indicate the 10th and 90th percentiles. Right: Similar to the left panel, but this time we plot the [C II]/FIR luminosity ratio as a function of the FIR surface brightness (Σ_FIR). The best fit to the data at z ≈ 0 (Lutz et al. 2016; Herrera-Camus et al. 2018b) and z ≈ 3 − 6 (this work; Eq. (2)) are shown as beige and gray lines, respectively.