Evolution of the t_depl and f_mol = M_mol/M_stars with redshift. The background blue contour levels represent the distribution of galaxies from the PHIBSS1/2 compilation (Tacconi et al. 2013, 2018). As a reference in redshift, we also show as green contours the distribution of galaxies detected in CO line emission at z < 0.5 from the PHIBSS1/2 compilation (e.g., from xCOLDGASS, GOALS, and EgNOG surveys). The solid lines show the expected evolution of t_depl and f_mol with redshift, based on previous targeted observations of star-forming galaxies. A fixed conversion factor α_CO = 3.6 (K km s⁻¹ pc²)⁻¹ has been assumed for the ASPECS CO sources. The comparison sample uses a metallicity-based prescription for this parameter. Typical values will range between α_CO = 2–5 M_⊙ (K km s⁻¹ pc²)⁻¹ for the ASPECS CO sources.