Redshift dependence of the molecular gas fraction. Upper panel: The molecular gas fraction f_mol (= M_mol/M_*) as a function of redshift for the SS, HI-FR galaxies and the xCOLDGASS sample (only detections in CO for xCOLDGASS in order not to overload the figure). Lower panel: The molecular gas fraction f_mol for the SSs has been adjusted to z = 0 following the nonlinear z-dependence found by Tacconi et al. (2020, their Table 3). f_mol for xCOLDGASS and HI-FR are the same as in the upper panel.