Models of young powerful radio sources

Observations of compact symmetric double sources suggest that these objects with physical scales of order tens of parsecs to kiloparsecs are young radio active galactic nuclei. There is, in general, a striking similarity between the structures of these compact objects and the structures of large evolved radio galaxies although the latter are two to three orders of magnitude larger. This has led to the use of self-similar models of the evolution of radio sources as a framework for interpreting observational data. However, the assumptions on which the self-similar models are based become increasingly less valid on the physical scales which are probed by the observations of the smallest sources. In this paper, the dynamics of sources on these small scales is examined and a model developed which extends earlier work in a self-consistent way to small physical scales. The limit of applicability of the models is identified as is the transition from an early evolutionary phase to the self-similar phase of expansion.