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 field galaxies dominates the infrared luminosity in the local volume. Galaxies with clear radio-excess (defined as having at least 5 times larger radio flux over expected from FIR) are identified as hosting a radio AGN, and they account for only about 1% of the whole sample. This fraction increases to 10% among the more luminous galaxies with L-1.4GHz greater than or equal to 10(23) W Hz(-1) (equivalently L-60 mu m greater than or equal to 10(11) L.), however. 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-60 mu m greater than or equal to 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 ultraluminous infrared galaxies which lack clear evidence for an AGN, such as Arp 220.