α_CO plotted against galactocentric radius normalized by R₂₅. Top panel:α_CO(R) obtained by multiplying the empirical normalized average profile of α_CO adopted from Sandstrom et al. (2013) by Milky Way α_CO, MW = 4.4 M_⊙ pc⁻² (K km s⁻¹)⁻¹; the errors are also from Sandstrom et al. (2013) and the black curve is the best-fit function. Bottom panel:α_CO(Z, Σ_⋆), marked in gray, derived from Eq. (8), which considers the decrease in α_CO driven by higher metallicity (Z) and stellar surface density (Σ_⋆); the black curve denotes the mean α_CO for a given radius.