8.4GHz observations of compact starbursts

Twenty-five contain diffuse radio sources obeying the FIR-radio correlation; these are almost certainly starburst galaxies. Fourteen other galaxies have nearly blackbody FIR spectra with color temperatures between 60 and 80K so their (unmeasured) FIR angular sizes must exceed approximately 0.25-arcsec, yet they contain compact (but usually resolved) radio sources smaller than this limit.

The unique radio and FIR properties of these galaxies can be modeled by ultraluminous nuclear starbursts so dense that they are optically thick to free-free absorption at {nu}~1.49GHz and dust absorption at {lambda}~25{mu}m. Only one galaxy (UGC 08058 = Mrk 231) is a dominated by a variable radio source too compact to be an ultraluminous starburst; it must be powered by a 'monster'.