Conversion factor maps. Top: CO(2–1)-to-CO(1–0) line ratio (R₂₁) map computed from CO(2–1) observations (PHANGS–ALMA) and CO(1–0) observations (ALMA science verification program) at a common 4″resolution. Middle: CO-to-H₂ conversion factor (α_CO) following the prescription from Bolatto et al. (2013) (Equ. A.2) using metallicities (bottom panel) from PHANGS–MUSE optical recombination lines observations (Williams et al. 2022).