Infrared photometry of the nuclei of early-type radio galaxies

Abstract

The authors have obtained near- and mid-infrared photometry of the nuclei of both strong and weak radio galaxies. The implication of these data are that a relatively small percentage, ≅15 per cent, of radio ellipticals have strong infrared emission. Although as a class they do have a 10-μm flux which is statistically greater than zero, especially the optically brightest, a large fraction of this excess can be attributed to starlight from the galaxy. Those galaxies which are detected at 10 μm tend to be bright at longer infrared wavelengths, and have a spectrum which is flatter than that typical of dust-lane ellipticals. Unlike Seyfert galaxies, there is no correlation between 10-μm flux and radio flux; the infrared fluxes for the compact active radio galaxies require a spectrum which turns over between 5 GHz and 10 μm. Only the galaxies with strongest 10-μm radiation have near infrared colours which are significantly redder than the norm.