Compact starbursts in ultraluminous infrared galaxies

The 40 ultraluminous [log(L_FIR_/L_sun_) >= 11.25] galaxies in the IRAS Bright Galaxy Sample of sources stronger than S = 5.24 Jy at λ = 60 microns were mapped with ~0.25" resolution at 8.44 GHz. 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 60 K <~ T_c_ <~ 80 K so their (unmeasured) FIR angular sizes must exceed θ >~ 0.25", 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 (τ >~ 1) to free-free absorption at ν~1.49 GHz and dust absorption at λ~25 microns. Only one galaxy (UGC 08058 = Mrk 231) is dominated by a variable radio source too compact (<~1 pc) to be an ultraluminous starburst; it must be powered by a "monster."