Line ratio versus metallicity. Left two panels: metallicity maps of NGC 3627 and NGC 4321 from Williams et al. (2022) based on PHANGS-MUSE (Emsellem et al. 2022) data. Boxes are regions as defined in Figs. 1 and 2. Right panel: R_C I/CO versus metallicity scatter plot for regions in NGC 3627 and NGC 4321. The bottom axis represents the metallicity in solar units, top axis is the metallicity in 12 + log(O/H), left-axis is the line ratio of [C I](1−0) and CO(2−1) in K km s⁻¹ units, and right-axis is the ratio of fluxes in erg s⁻¹ cm⁻² sr⁻¹ units. The left and right axes are matched by a factor of (ν_[C I]1−0/ν_CO1−0)³ × R₂₁, where we adopt R₂₁ = 0.65 (Leroy et al. 2022). Data points are the mean metallicity and R_C I/CO in each region shown in left panels, with error bars indicating the typical uncertainty in R_C I/CO, and vertical color shading indicating the 16th to 84th percentiles of the pixels’ R_C I/CO distribution inside each region. The Bolatto et al. (2000a) empirical fitting and Bolatto et al. (1999) model prediction lines are overlaid. The three outliers in NGC 3627 with R_C I/CO ∼ 0.19, 0.28, and 0.35 are regions 20, 21, and 22, respectively. The outlier point in NGC 4321 with R_C I/CO ∼ 0.06 is its center is region 1.