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5.1. What are the Ingredients?

In Section 4, it was argued that the sidedness, transverse brightness profile and field structure properties of strong-flavor jets are best explained if they have relativistic bulk velocities. This leads to the following identifications for the structures identified in Section 2:

Strong-flavour jets are highly-collimated, relativistic (and therefore necessarily supersonic) flows. They occur in the nuclei of most, if not all radio galaxies and quasars.

Central components are the optically-thick bases of strong-flavor jets.

Weak-flavour jets are supersonic (or, at least, transonic), sub-relativistic flows. They are formed by the smooth deceleration of strong-flavor jets.

Hot-spots in FRII sources are the post-shock flows formed when a highly-supersonic (and, therefore, nearly always strong-flavor) jet terminates by collision with the surrounding medium. The flow speed of material emerging from a hot-spot may be relativistic in some cases, particularly in the most powerful sources. Hot-spots in FRI sources such as wide-angle tails may be formed by a different mechanism, but also occur where a strong-flavor jet disrupts.

Bridges are subsonic backflows from the end of a jet of either flavor.

Tails are outwardly-moving subsonic flows.

5.2. FRI Sources

5.3. FRII Sources

5.4. Concluding Remarks

The very simple picture presented here is conventional in many respects. The essential new point is the suggestion that all strong-flavor jets and jet bases are relativistic and that the marked differences between the strong and weak flavors result directly from the importance or otherwise of beaming and aberration effects. The initial speeds of all kiloparsec-scale jets are then very similar and the differences in radio luminosity and morphology depend on the jet's power and the environment through which it propagates. In particular, the division between FRI and FRII sources may be determined by the ability of a jet to decelerate smoothly without disruption.


I thank the ST ScI for travel expenses, Geoff Bicknell, Paola Parma and Alan Bridle for their thoughts and the editors for their extreme patience.

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