Empirical relations used to produce the [CI] line fluxes. Upper panel: relation between the L_{[CI](1 − 0)}/L_IR and L_{CO(4 − 3)}/L_IR ratios. The filled blue circles and the red filled squares are, respectively, the low-z and high-z galaxies compiled by Valentino et al. (2020b). The solid line is our best-fit of this correlation used to generate [CI](1–0) fluxes in our simulation. Lower panel: same figure for the relation between the L_{[CI](2 − 1)}/L_{[CI](1 − 0)} and L_{CO(7 − 6)}/L_{CO(4 − 3)} ratios.