Mean trend for the X_CO factor evolution as a function of W_CO as predicted by simulations, adapted from Figure 6 of Bertram et al. (2016) and Figure 10 of Gong et al. (2020). The effect of the turbulence is taken into account in Bertram et al. (2016) by varying the ratio of kinetic and potential energies (dotted curves). The dashed curves correspond to the reference functions from Gong et al. (2020), noted $f\left(W_{{12}_{\text{CO}}}^{10}\right)$ in Eq. (6). A power-law correction of index η can be applied to this curve in order to mimic the effect of the turbulence, as proposed in Eq. (8).