Model calculations in arbitrary units. Left column: t_eff = t_sync without diffusion; middle column: without diffusion; right column: with diffusion. Upper row: radio continuum surface brightness at 4.85 GHz as a function of the star formation rate per unit area; middle row: spectral index as a function of the ratio between the radio continuum surface brightness and the star formation rate per unit area; lower row: left: star formation rate per unit area; middle and right: radio continuum surface brightness at 4.85 GHz (contours) on radio-dim (red) and radio-bright (blue) regions.