THE RADIO STRUCTURES OF ULTRALUMINOUS IRAS GALAXIES

We present multifrequency radio observations of five ultraluminous IRAS galaxies, Arp 220, Mkn 273, UGC 05101, IRAS 08572 + 3915 and IRAS 15250 + 3609, made with the VLA in three array configurations; a MERLIN map of UGC 05 1 01 at 1.66 GHz; and integrated flux measurements at 151 MHz with the CLFST for all five galaxies. Although the radio emission from these objects tends to be concentrated within the central 1 kpc, it is not from a compact active nucleus, and there is evidence for extended radio emission in each case. Three of the galaxies show low-frequency turnovers in their spectra which may be caused by thermal absorption of their nonthermal spectra. We conclude that in each object almost all of the radio and far-infrared emission is associated with circum-nuclear or disc-wide star formation and not with active nuclei, although our observations do not preclude the existence of active nuclei in these galaxies.