Radio Agns among Luminous Infrared Galaxies

An analysis of radio and FIR emission in over 1500 IRAS selected galaxies produces a good linear correlation between radio and FIR luminosity, indicating that star formation in normal eld galaxies dominates the infrared luminosity in the local volume. Galaxies with clear radio-excess (deened as having at least 5 times larger radio ux over expected from FIR) are identiied as hosting a radio AGN, and they account for only about 1% of the whole sample. This fraction increases to 10% among the more luminousever. The characteristic mid-IR excess of a Seyfert nucleus is ubiquitously present among the radio-excess objects, suggesting that mid-IR excess is a robust tracer of an AGN despite the high mid-IR opacity. We conclude that about 30% of the luminous infrared galaxies (L 60m 10 11 L) host an AGN based on the mid-IR excess, and about 40% of the mid-IR excess AGNs also host a radio AGN. A VLA imaging survey of a distance limited sample of IR luminous galaxies has revealed the presence of 100 kpc scale giant radio plumes in 3 out of 9 cases (Mrk 231, Mrk 273, NGC 6240). Their large spatial extent, energetics, and presence of a powerful AGN in each case suggests that an AGN is the power source. Such plumes are not detected in other ultralu-minous infrared galaxies which lack clear evidence for an AGN, such as Arp 220.