Radio sources in dense groups

In order to investigate effects of galaxy environment on radio emission, 88 compact groups of galaxies were observed, with densities as high as one million galaxies per cu Mpc, at a wavelength of 18 cm at the VLA. Forty-one sources were found above a flux limit of 1.5 mJy, with roughly equal frequency in all galaxy types. Elliptical galaxies with detectable radio emission are almost always the first-ranked galaxies in optical luminosity. They are less luminous than typical rich cluster radio galaxies, are coincident with the optical nucleus, and do not show extended structure. Radio sources occurring in spiral galaxies show no strong preference for optical rank. It is argued that this is strong evidence that the galaxy environment on large scales plays a crucial role in the development and fueling of nuclear radio sources.