Compact radio emission from warm infrared galaxies

In this paper, we present a comparison between the optical spectroscopic data and the incidence of compact radio emission for a sample of 60 warm infrared galaxies. We find that 80% of optically classified active galactic nucleus (AGN)-type galaxies contain compact radio sources, while 37% of optically classified starburst galaxies contain compact radio sources. The compact radio luminosity shows a bimodal distribution, indicating two populations in our sample. The majority of the higher radio luminosity class (L > 10⁴ L⊙) are AGNs, while the majority of the lower radio luminosity class (L < 10⁴ L⊙) are starbursts. The compact radio emission in the starburst galaxies may be due to either obscured AGNs or complexes of extremely luminous supernovae such as that seen in Arp 220. The incidence of optically classified AGNs increases with increasing far-infrared (FIR) luminosity. Using FIR color-color diagrams, we find that globally the energetics of 92% of the galaxies in our sample are dominated by starburst activity, including 60% of galaxies that we find to contain AGNs on the basis of their optical classification. The remainder are energetically dominated by their AGNs in the infrared. For starburst galaxies, electron density increases with dust temperature, consistent with the merger model for infrared galaxies.