Pixel-based relation between the 8-to-24 μm intensity ratio and the PAH mass fraction of the Milky Way-like galaxy simulation at t = 0.5, 1.0, 3.0, and 10 Gyr from top left to bottom right. The color represents the density-weighted mean intensity at 1000 Å relative to the Milky Way value as shown by the color bar on the top. Blue dashed lines indicate the best-fit relation given by Eq. (7), and the legends show the values of the coefficients k and c.