2.1. The Standard Model
When imaged with adequate sensitivity and resolution powerful (FRII) double radio galaxies always show four basic morphological components: at the center of the parent optical galaxy is a compact, typically flat spectrum, high surface brightness `core' component. Extending from this compact component to the source extremities are highly elongated radio emitting structures, or `jets'. The jets usually end at, or point towards, high surface brightness `hotspots' at the extremities of the radio source. The region in between the radio hotspots is filled with extended, low surface brightness, often filamentary emission, known as `radio lobes'. These four components are all clearly evident on the 5GHz radio image of Cygnus A made with the VLA (Fig. 2).
Figure 2. A greyscale representation of the image of Cygnus A at 5 GHz with 0.4" resolution made with the VLA (courtesy R. Perley).
The standard physical model explaining and relating these four morphological structures is known as the `jet model' for powerful radio galaxies, as first suggested in Longair et al. (1973) and Hargrave and Ryle (1974), and worked out in detail in Blandford and Rees (1974) and Scheuer (1974). A seminal review of this topic can be found in Begelman et al. (1984), and we briefly summarize the basics of this model.
The radio core corresponds to the `central engine' - the ultimate source of energy responsible for the double radio structures. Unfortunately our understanding of the physics of central engines in AGN remains limited, mostly due to the lack of direct observational constraints. For reviews of central engines in AGN see Begelman et al. (1984), Begelman (1993), Miller (1985), and Wiita (1991). The one certain fact about central engines is that they generate a tremendous amount of energy, >> 1045 ergs sec-1 in a very small volume (<< 1 pc radius). Some, or perhaps much, of this energy can be released in the form of highly collimated, supersonic, and probably relativistic, outflows of plasma and magnetic field - the radio `jets'. For reviews of various aspects of jets in powerful radio galaxies see Bridle and Perley (1984), Bridle and Eilek (1985), Zensus and Pearson (1990), Hughes (1991), Burgarella et al. (1993), Zensus and Kellermann (1994), and Röser and Meisenheimer (1995).
The jets in FRII sources propagate relatively unhindered until they terminate in a strong shock on impact with the external medium. At this point the jets convert some, perhaps most, of their bulk kinetic energy into relativistic particles (through first order Fermi acceleration), and magnetic fields (through simple shock compression, or more complex dynamo processes in the turbulent post-shock flow). This post-jet shock fluid emits copious radio synchrotron radiation resulting in the high surface brightness radio `hotspots'. Reviews of physical processes in hotspots can be found in Röser and Meisenheimer (1989). The high pressure shocked jet material then expands out of the hotspot transversely inflating a synchrotron emitting `cavity' in the ambient medium of waste-jet material - the radio `lobe'.
The outline above applies to the radio emitting structures. A second aspect of this model is the effect of the radio source on the ambient medium. Fig. 3A shows a schematic of the effect an expanding radio source will have on the ambient medium on large scales, while Fig. 3B shows a detail of the terminal jet shock structure. At the jet terminus two shocks are formed: the jet shock, or Mach-disk, which effectively stops the incoming jet, and the standoff, or bow, shock which acts to accelerate and heat the ambient medium. The two shocked fluids (jet and ambient) meet in pressure balance along a contact discontinuity. Observational evidence suggests that the contact discontinuity is largely stable to mixing (Carilli, Perley, and Dreher 1988).
Figure 3a and b. A schematic representation showing the effect an expanding radio source will have on the external medium. Fig. A is reproduced from Begelman and Cioffi (1989), and shows the expected large-scale distribution of shocked ambient gas enveloping the radio lobes. Figure B is reproduced from Smith et al. (1985) and shows a detail of the expected double-shock structure at the jet terminus. The interface is the contact discontinuity between shocked jet material and shocked intracluster material (ICM). The beam shock and cap correspond to the radio hotspots, and the cocoon corresponds to the radio lobe.
Hence the overall picture is one of a radio source being enveloped by a `sheath' of shocked ambient medium. If the jet direction is roughly constant over time the radio lobe grows as a roughly `cigar-shaped' cavity with the principal active surface being associated with the high radio surface brightness regions at the lobe extremities. In this model the shocked intracluster medium forms a thin dense sheath in the vicinity of the hotspots (since this is the location of the `driving piston'), and a much broader, less dense sheath in the vicinity of the tails of the radio lobes.