Pixel-based relation between the 8 μm-to-total IR intensity ratio and the PAH mass fraction of the Milky Way-like and NGC 628-like galaxies using all snapshots at t = 0.5, 1.0, 3.0, and 10 Gyr (left and right panels, respectively). The color represents the density-weighted mean intensity at 1000 Å relative to the Milky Way value as indicated by the color bar. Blue dashed lines indicate the best-fit relation of Eq. (B.1), and the legend indicates the best-fit parameter.