Dependence of IR8 (=/L[8 μm]) with Σ_IR (=/(2  ×  π)), the IR surface brightness. Galaxies to the right of the vertical dotted line are considered to be compact star-forming galaxies (Σ_IR ≥ 3    ×  10¹⁰ L_⊙ kpc^-2). Galaxies above the horizontal dotted line exhibit IR8 ratio that is two times larger than that of main sequence galaxies, which follow the Gaussian IR8 distribution shown in Fig. 9). The sliding median is shown with open blue dots. It is fitted by the solid line in red and its 16th and 84th percentiles are fitted with the dashed red lines (see Eq. (8)).