Line ratio versus mean ISRF intensity ⟨U⟩ measured from SED fitting to the near- to far-IR (Spitzer, Herschel) data. Left four panels: ⟨U⟩ maps of NGC 3627, NGC 4321, NGC 1808 and NGC 7469, and as described in Sect. 3.3 are obtained from the works done by Chastenet et al. (in prep.) as a continued effort of Chastenet et al. (2021). Right panel: scatter plot of R_C I/CO versus ⟨U⟩ at a spatial resolution of ∼18″ in the manually-selected apertures shown in the left panels. We computed a beam-weighted mean R_C I/CO and a center pixel R_C I/CO for each aperture, displayed as the solid and open symbols, respectively. Error bars are the corresponding beam-weighted mean or central pixel R_C I/CO uncertainties in each region. The region diameter (∼18″) corresponds to the coarsest resolution of the Herschel data used in the SED fitting. The gray dotted line shows R_C I/CO = 0.07 U^0.25 as a guiding line. The colored dotted contours (labeled 0.1−0.5) are the model contours of R_C I/CO computed from the PDR Toolbox (using the latest wk2020 model and solar metallicity; Kaufman et al. 2006). We convert G₀ from the model contour to the x-axis U by multiplying with a factor of 0.88 (Draine et al. 2007). The right y-axis indicates the PDR model’s n_H2, whose range is manually adjusted so that the model contours roughly match the left x-axis. The PDR model contours show little dependence of R_C I/CO on U in the horizontal direction but a strong correlation with n_H2 as seen by the vertical gradient.