Time evolution of pixel-based relation among the 8 μm-to-total IR intensity ratio, gas-phase metallicity, and hydrogen surface density for the Milky Way-like galaxy simulation. The panels show the relation at t = 0.5, 1.0, 3.0, and 10 Gyr from top left to bottom right. The color bar indicates the hydrogen surface density of each pixel. In each panel, the time evolution of the entire distribution over the four epochs is also indicated by the purple dashed lines. The purple star symbols represent the global intensity ratio and metallicity of the galaxy at t = 0.5, 1.0, 3.0, and 10 Gyr from the lower left to the upper right in each panel (the symbol transparency decreases) and are connected by the dashed line. We note that the intensity ratio and metallicity are the intensity- and mass-weighted values, respectively. The grey line shows the evolutionary path in the one-zone dust evolution model by Hirashita & Murga (2020) under the condition of ƒ_dense = 0.5, τ_SF = 5 Gyr, and U_MW = 1.