2.1. General observational constraints on models
It seems likely that the simplest and most important extended sources to understand are the strong "active" doubles like Cygnus A, characterised by hot spots at their outer edges. In at least three such sources, aligned radio structure has been discovered in the nucleus. This indicates that energy is still being supplied to the extended components and that the source axis can be accurately "remembered". Further corroborative evidence for this view is provided by the "tunnels" sometimes seen in some relaxed sources, the jets in M87 and 3C 273, and the prevalence of similar aligned structure within active nuclei. It is natural to interpret the hot spots as the place where this energy is in some sense dissipated, probably by means of the acceleration of relativistic particles and the amplification of magnetic field. The tails extending back towards the parent galaxy can then be regarded as the glowing embers from earlier activity.
We shall here concentrate on describing the so-called "beam" model. When a detailed model of this general type was first proposed in 1971, the only arguments in its favour were somewhat circumstantial: for instance, it had the appealing feature that the energy content of extended sources could accumulate over their entire lifetime, which necessitated power outputs of only 1046 erg s-1 (whereas other ideas current at that time had to hypothesise, ad hoc, a short-lived outburst of vastly higher power); and it obviated the problem of adiabatic losses which occur if "plasmoids" generated in the galactic nucleus move bodily out into the relatively diffuse components. But three other categories of evidence have subsequently supported the beam model:
(i) There are numerous cases of "bridges" or "tails" linking the hot-spots to the central galaxy, and of continuing non-thermal activity in the galactic nuclei themselves.
(ii) Higher-resolution maps show that the electron lifetimes in the hot-spots of Cygnus A, 3C 236, etc. are shorter than the likely source lifetime (and perhaps shorter than the light travel time from the centre). This indicates the need for continuing re-plenishment or in situ acceleration of electrons in the hot spots.
(iii) The evidence for compact central radio components aligned with the overall source axis, indicates beams with a well-defined and persistent orientation.
Some further recent improvements in our knowledge that are consistent with beam models (or raise difficulties for alternative theories) but cannot be claimed as adding unambiguous support, include:
(iv) Better upper limits on the gas density in extended components.
(v) Continuing failure to find compact ( << 1") features within extended components away from the galactic nucleus.
(vi) Evidence that the source axis may in some cases be correlated with the rotation axis for the galaxy.
(vii) Tentative evidence for optical emission from one or two radio components.
Although the beam model seems - in broad outline - in accord with the data we would warn against adopting it too uncritically, or favouring it too strongly over some alternative ideas. Few aspects are yet worked out in adequate detail; the supporting evidence is far from overwhelming; there is in any case no reason why all radio sources should necessarily involve the same mechanism; and the history of extragalactic radio astronomy has often vindicated the sceptic.
The general rough estimates of the energy content in radio components are the same in beam models as in any theory which attributes the radio emission to a synchrotron-type process. We shall not repeat these well-known considerations here.