12.5.10. Evolution of the Radio Sources
As a result of the discovery and continued observations of the intensity variations in radio galaxies and quasars, it is becoming increasingly clear that, on a cosmic time scale, the generation of particles occurs during very short times and is presumably the result of repeated violent events in the nuclei of galaxies and in quasars. This essentially precludes any statistical process such as Fermi acceleration, whose time scale is measured in millions of years. The problem of the energy source is made even more difficult because of the large energy loss expected when the cloud of relativistic particles expands.
However, even aside from the question of the source of energy and its conversion to relativistic particles, there is the problem of the formation and evolution of the extended radio sources. In particular, how are the clouds of relativistic particles confined, and what determines the characteristic double or multiple shape?
One particular problem has been the need to explain the extremely fine structure found in the outer parts of some extended sources such as Cygnus A, or the existence of sources with very high ratio of component separation to component size. If there is a cold dense plasma to contain the relativistic particles, then the minimum energy requirements are magnified as a result of the necessary increase in the kinetic energy. On the other hand, the initial presence of a large kinetic energy may provide a source of continuous energy input to balance that lost by expansion. However, it is difficult to see any easy way for the kinetic energy of the cold plasma to be transformed into energy accelerating the relativistic particles.
One interesting suggestion has been that rather than accelerating the relativistic particles themselves, the galactic nuclei and quasars expel massive coherent bodies which then explode at some distance from the origin and produce the particles in situ (e.g., Burbidge, 1972). But there is no experimental evidence that this occurs since all of the observed very compact opaque sources, and all of the variable sources, are coincident with quasars or nuclei. Thus, it appears that unless the number of coherent ejections is large (N > 1000) so that individual events are not seen, the relativistic particles in each source are in fact generated at a common point and diffuse out to form two or more extended clouds of particles. It has therefore been widely assumed that the individual components are confined by the inter-galactic gas or intergalactic magnetic fields.
The density of any intergalactic gas deduced from measurements of the X-ray background is too small for confinement by static pressure, but De Young and Axford (1967) have suggested that the internal pressure of the ejected "plasmons" can be balanced by the ram pressure of the inter-galactic gas. Wardle (1971) has noted that the ram pressure model provided a natural explanation for the power-surface brightness diagram discussed by Heeschen (1966). The required expansion velocities are 5 to 10 that of light, so that the characteristic ages are only 105 to 106 years. This is much less than deduced from the energy arguments (see Section 12.5.7). This shorter time scale would also make it easier to understand the lack of any high-frequency cut-off in the radio source spectra discussed in Section 12.5.3.
Since the density of any intergalactic gas is expected to vary as (1 + z)3, this model also provides a natural explanation for the small size found by Miley (1971) for double radio sources at large redshifts. The presence of extended tails observed for several radio galaxies (Miley et al., 1972) is also predicted by the model (De Young, 1971). The rather sharp outer boundaries found in many of the resolved sources lend support to the ram pressure model, although the required gas density or magnetic field strengths are uncomfortably large (e.g., Burbidge, 1972). However, the possibility of sufficient gas being present in clusters or being ejected from the galaxy itself does not seem unlikely, and the observation that double radio sources found in clusters tend to be smaller than those not in clusters provides indirect evidence for intergalactic matter in clusters (De Young, 1972).
A model describing the evolution of double radio sources has been developed by Ryle and Longair (1967). They explain the ratio of observed intensity and separation from the parent object as the effect of the differential light travel time and other relativistic effects on two identical objects expanding with highly relativistic velocities.