Formation of double radio source structures and superluminal expansion

Abstract

If the shock wave formed by an explosion in the nucleus of an active radio galaxy fragments, the trajectories of the individual plasma fragments will be naturally collimated into two distinct directions, thus resulting in the familiar double-lobed radio source geometry. The collimation and focusing of kinetic energy is the result of anisotropic effects of ram pressure on the individual plasmoids as they traverse the disklike distribution of gas in the active galactic nucleus.Calulations of the evolution of the spatial brightness distribution from an initially isotropic ejection of relativistic plasma fragments are performed. These calculations demonstrate that an apparent superluminal separation of the two halves of the double-lobed brightness distribution will be observed. A brief comparison is made between the theoretical results and VLBI observations of those compact double sources which exhibit ''superluminal'' expansion.