Compact radio emission from warm infrared galaxies

In this paper, we present a comparison between the optical spectroscopic da ta and the incidence of compact radio emission for a sample of 60 warm infr ared galaxies. We find that 80% of optically classified active galactic nuc leus (AGN)-type galaxies contain compact radio sources, while 37% of optica lly classified starburst galaxies contain compact radio sources. The compac t radio luminosity shows a bimodal distribution, indicating two populations in our sample. The majority of the higher radio luminosity class (L > 10(4 ) L.) are AGNs, while the majority of the lower radio luminosity class (L < 10(4) L.) are starbursts. The compact radio emission in the starburst gala xies may be due to either obscured AGNs or complexes of extremely luminous supernovae such as that seen in Arp 220. The incidence of optically classif ied AGNs increases with increasing far-infrared (FIR) luminosity. Using FIR color-color diagrams, we find that globally the energetics of 92% of the g alaxies in our sample are dominated by starburst activity, including 60% of galaxies that we find to contain AGNs on the basis of their optical classi fication. The remainder are energetically dominated by their AGNs in the in frared. For starburst galaxies, electron density increases with dust temper ature, consistent with the merger model for infrared galaxies.