NGC 4501 model (arbitrary units). Upper left: radio continuum surface brightness at 4.85 GHz as a function of the star formation rate per unit area; upper right: star formation rate per unit area; lower left: spectral index as a function of the ratio between the radio continuum surface brightness and the star formation rate per unit area; lower right: radio continuum surface brightness at 4.85 GHz (contours) on radio-dim (red) and radio-bright (blue) regions.