Environment dependence of R₃₂. The ratio (with uncertainty lower than ~25%) is correlated with the local intensity of CO(3–2) and CO(2–1) in panels (a) and (b), respectively. In panel (c), R₃₂ as a function of the far-infrared (FIR)color of the Spitzer MIPS bands (Gordon et al. 2011), and in panel (d), R₃₂ as a function of the TIR surface brightness from Spitzer and Herschel bands (Gordon et al. 2011, 2014). The red and black dashed lines in all panels indicate the median value of R₂₃ and the best fit, respectively. The Spearman’s correlations (ρ_S) and the best-fit equations (in log–log scale) are located in the right bottom corners.