Bottom panel: evolution of the ⟨f_gas⟩ of quiescent galaxies as a function of redshift. The various symbols with error bars or upper-limit arrows show the median f_gas of the QG and pSB samples. The solid and hatched light gray regions trace the envelope of the best-fit models with constant f_g, 0, within the range of median stellar masses of the samples, and its predicted 1σ scatter, respectively. The dark dashed gray lines show the best-fit envelope for the fractional case model and the horizontal black line shows the best-fit f_g, 0, s for the constant case model. For comparison, the evolution of f_gas of 5 × 10¹⁰ M_⊙ MS galaxies, as parameterized by S14, is shown as a tan line and its scatter envelope is shown as a solid cream region. Top panel: median residuals of the fits with constant and fractional f_g, 0 (solid and clear symbols, respectively, the latter having been offset for clarity) for the same samples. The error bars show the dispersion of residuals. Smaller symbols (Sargent et al. 2015; Rudnick et al. 2017; Bezanson et al. 2019) indicate single objects.