Radio-Excess IRAS Galaxies: PMN/FSC Sample Selection

Abstract

A sample of 178 extragalactic objects is defined by correlating the 60{μ}m IRAS Faint Source Catalog (FSC) with the 5 GHz Parkes-MIT-NRAO(PMN) catalog. Of these, 98 objects lie above the radio/far-infraredrelation for radio-quiet objects. These radio-excess galaxies andquasars have a uniform distribution of radio excesses, with no evidencefor a radio-loud correlation equivalent to the well-known radio-quietradio/far-infrared relation. The radio-intermediate objects appear to bea new population of active galaxies not present in previousradio/far-infrared samples chosen using more stringent far-infraredcriteria. The radio-excess objects extend over the full range offar-infrared luminosities seen in extragalactic objects, fromlow-luminosity galaxies with {ν}L_{nu}(60{μ}m){\lt}10^{9} L_{solar} to ultraluminous infraredgalaxies with {ν}L_{nu}(60 {μ}m){\gt}10^{12}L_{solar}. Objects with small radio excesses are more likely tohave far-infrared colors similar to starbursts, while objects with largeradio excesses have far-infrared colors typical of pure AGNs. Some ofthe most far-infrared luminous radio-excess objects have the highestfar-infrared optical depths. These are good candidates to search forhidden broad-line regions in polarized light or via near-infraredspectroscopy. Some low far-infrared luminosity radio-excess objectsappear to derive a dominant fraction of their far-infrared emission fromstar formation, despite the dominance of the AGN at radio wavelengths.Many of the radio-excess objects have sizes likely to be smaller thanthe optical host but show optically thin radio emission, rather thanflat radio spectra indicative of compact quasar cores. We draw parallelsbetween these objects and high radio luminosity compact steep-spectrumand gigahertz peaked-spectrum objects. Radio sources with thesecharacteristics may be young AGNs in which the radio activity has begunonly recently. Alternatively, high central densities in the hostgalaxies may be confining the radio sources to compact sizes. We discussfuture observations required to distinguish between these possibilitiesand determine the nature of radio-excess objects.