Gas fraction, f_gas = M_tot gas/(M_star + M_tot gas), within r₂₅ as a function of the morphological stage, from T = 1 to T = 10, assuming a constant X_CO (Bolatto et al. 2013, filled green dots) and a metallicity-dependent X_CO (Amorín et al. 2016, empty red squares). The symbols represent the mean values and their uncertainties (expressed in logarithm) in bins of ΔT = 1, except for the morphological stages T = 8, 9, 10 that are binned together because of a smaller number of later-type galaxies than earlier-type ones.