Brightness temperature versus radio excess. q was determined using IR luminosities from the CyE95 (left) and CiS12 (right) models. The solid, dashed, and dotted blue lines indicate the maximum brightness temperature from starbursts for a synchrotron spectral index α of −0.7, −0.8, and −0.9, respectively, as per Condon et al. (1991). Points above these lines cannot be driven by pure star formation.