Radio-ExcessIRASGalaxies: PMN/FSC Sample Selection

A sample of 178 extragalactic objects is defined by correlating the 60 micronIRAS FSC with the 5 GHz PMN catalog. Of these, 98 objects lie above theradio/far-infrared relation for radio-quiet objects. These radio-excessgalaxies and quasars have a uniform distribution of radio excesses and appearto be a new population of active galaxies not present in previousradio/far-infrared samples. The radio-excess objects extend over the full rangeof far-infrared luminosities seen in extragalactic objects. Objects with smallradio excesses are more likely to have far-infrared colors similar tostarbursts, while objects with large radio excesses have far-infrared colorstypical of pure AGN. Some of the most far-infrared luminous radio-excessobjects have the highest far-infrared optical depths. These are good candidatesto search for hidden broad line regions in polarized light or via near-infraredspectroscopy. Some low far-infrared luminosity radio-excess objects appear toderive a dominant fraction of their far-infrared emission from star formation,despite the dominance of the AGN at radio wavelengths. Many of the radio-excessobjects have sizes likely to be smaller than the optical host, but showoptically thin radio emission. We draw parallels between these objects and highradio luminosity Compact Steep-Spectrum (CSS) and GigaHertz Peaked-Spectrum(GPS) objects. Radio sources with these characteristics may be young AGN inwhich the radio activity has begun only recently. Alternatively, high centraldensities in the host galaxies may be confining the radio sources to compactsizes. We discuss future observations required to distinguish between thesepossibilities and determine the nature of radio-excess objects.