Annu. Rev. Astron. Astrophys. 1988. 36: 539-598 Copyright © 1998 by Annual Reviews. All rights reserved

5. JET TERMINATION: THE COCOON

Jets terminate in extended radio lobes, where they release their energy and momentum into the ambient intergalactic medium through complex physical processes; see for instance the western radio lobe of Cygnus A for a typical example (Carilli et al 1996). A phenomenological analysis of the physics of this region is given below as a starting point for presenting the nonlinear models.

5.1. Hot Spots

Parker, in his model for solar wind, suggested that the supersonic solar wind solution must be connected to the subsonic dynamics of the interstellar medium through a "termination shock" matching the asymptotic wind to a breeze solution with P 0 (Parker 1958). In laboratory experiments of collimated jets, this is accomplished through a strong planar shock, or "Mach disk," that creates a localized high pressure region. In extragalactic jets, this feature is identified with the hot spots, compact overpressured features, that are especially evident in FR II jets; they are instead dim or absent in weak FR I sources, most likely because strong turbulent dissipation in the propagation phase reduces the momentum finally released at the termination shock.

The location of the jet head, in particular that of the hot spots, is defined by balancing the internal thrust and external medium ram pressure:

(19)

where L_j is the jet's kinetic luminosity, v_j the flow velocity, v_h the head's advancing velocity, and _j and _ext the jet's and ambient matter's density, respectively. Then, with = _ext / _j, we obtain the typical velocity at which the hot spots plough their way into the external medium:

(20)

i.e. light jets ( > 1) are decelerated more than heavy jets ( < 1). The distance travelled by the hot spot depends on the source's lifetime; it can be much larger than any stopping distance of a single plasmon.