Compact symmetric objects and the evolution of powerful extragalactic radio sources

Multifrequency radio observations of the compact symmetric objects 0108+388, 0710+439 and 2352+495, and high-resolution VLA observations of the primary hot spot in Cygnus A, are used to investigate the dependence on external density of the size, pressure, advance speed, and flux density of the hot spots in powerful extragalactic radio sources. The relationships derived are applied over the whole range of scales observed in these objects. These relationships lead directly to a unifying evolutionary model for powerful radio sources, in which compact symmetric objects evolve first into compact steep spectrum doubles, and then into large Fanaroff-Riley type II objects. We propose that this is the primary evolutionary track for powerful extragalactic radio sources. The observations strongly suggest that these objects evolve substantially in luminosity, with little variation in expansion speed between 10 pc and 200 kpc.