Interstellar Matter in Early-Type Galaxies. III. Radio Emission and Star Formation

IRAS measurements of the far-infrared emission from early-type galaxies (ellipticals and S0s) are compared with radio continuum data, including a recent sensitive 6 cm survey. For ellipticals the presence of long-wavelength IR emission (presumably an indicator of cold interstellar material) enhances the probability that the galaxy is a radio source and is also correlated with the strength of that source. These findings are consistent with the idea that active radio nuclei are due to black holes being fueled by accretion of gas. S0s show a wide variety of radio-far-infrared relations. Taking just those galaxies which are detected in both, the majority are found to have a similar ratio of IR to radio luminosity as has been found in spirals, and which is thought to be indicative of recent star formation. A few S0s have excess radio emission (most likely this is nonthermal emission from a nuclear source) and this too may be related to the infrared luminosity. Sensitive radio limits for several galaxies reveal another substantial population of S0s with moderately strong infrared emission unaccompanied by radio power. This IR emission is not closely correlated with either optical luminosity or far-infrared color.