Molecular gas mass fraction as a function dust-to-gas mass ratio, for each galaxy in the sample. The molecular gas is estimated assuming a constant MW α_CO = 4.4 M_⊙ pc⁻² (K km s⁻¹)⁻¹ (Bolatto et al. 2013 and R₂₁ = 0.7 (top panels), a radially constant α_CO which varies from galaxy to galaxy as found by Sandstrom et al. (2013) (middle-top panels), a constant α_CO = 4.4 M_⊙ pc⁻² (K km s⁻¹)⁻¹ throughout the galaxies except in the central areas, and a radially constant R₂₁ value varying from galaxy to galaxy (den Brok et al. 2021; Yajima et al. 2021; Leroy et al. 2022, middle-bottom panels), and a radially varying α_{CO(2 − 1)} as found by Chiang et al. (2024, bottom panels). The α_CO and R₂₁ values used for each panel are shown in the lower right corners. Pixels within the inner 0.15 R₂₅ area are plotted with open symbols. Uncertainties are shown with grey error bars.