Hot-spots in extragalactic radio sources: shocks and the distribution of energy

The physics of the hot-spots of extended extragalactic radio sources is discussed. Although the canonical model assumes that high-energy electrons are injected at the shock, consideration of high Mach number hydromagnetic shocks suggests that this is an over-simplification. Observational and experimental evidence indicates that such shocks break into macroscopic turbulence and that particle processes are inadequate to thermalize the flow energy. A model is proposed in which both particle and magnetic field energy are derived from these macroscopic motions. Waves, generated when the shock breaks, give rise to the large-scale magnetic field by dynamo action and steepen as they propagate, producing a distribution of secondary shocks within which electrons are accelerated to the threshold for Fermi acceleration. It is shown that, as a consequence of this, there will be approximate energy equipartition between waves, particles and large-scale magnetic field. (author)