Introduction

What are extragalactic radio sources?

We believe that extragalactic radio sources are interactions between large-scale jets and the hot, diffuse gas that surrounds elliptical galaxies. The interactions not only cause the hotspots, bridges, tails etc., but also the radio emission from the jets themselves. Radio sources should be thought of as processes rather than objects: the overall radio structure must change substantially on the shortest possible dynamical time scale, the sound-crossing time. Another way to put this is that, at least in the FR II sources, there is no steady-state description. This is the main reason why radio sources are much harder to understand than, say, main-sequence stars. On the other hand, the lack of equilibrium means that the structure of radio sources reflects their past history, so that in principle it should be much easier to deduce the life-cycle of radio sources than that of stars. At present, the most successful theoretical models concentrate on regions for which a local steady-state description is likely to be appropriate, notably the bases of jets, far from any end-effects, and in the co-moving frame of the hotspots, where the jets terminate.

It is worth emphasizing that in the standard model of FR II sources, and to a lesser extent FR Is, we only see half the story in the radio. As we shall see, it is usually assumed that synchrotron radiation is only emitted by plasma which has entered the system via the jet.