5. SUMMARY

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

• FRI jets have _j 1 close to their nuclei and are initially light.

• They decelerate on kiloparsec scales in a way determined by jet power and environment.

• A plausible mechanism for deceleration in twin-jet sources is entrainment of external material into a transonic flow (Begelman 1982; Scheuer 1982; Phinney 1983; Komissarov 1992). Mass loss from stars may also provide enough material. The jet can only decelerate in this way provided that it can be recollimated by the external pressure gradient. This sets an upper limit to the kinetic luminosity of the jet of 10^35-36 W for plausible galactic parameters (Phinney 1983; compare the estimates given earlier).

• Strong-flavor jet bases have _j 1 and are therefore one-sided. They are limb-brightened and have longitudinal apparent magnetic field because we see the boundary layer of a decelerating flow.

• Weak-flavor jets are transonic flows with _j << 1. They may well be turbulent, as suggested by Bicknell.

• Tails and bridges are subsonic. Tails are likely to be formed if the jet entrains enough material to bring the density contrast close to 1 or if buoyancy causes the central region of a bridge to be pushed outwards.

5.3. FRII Sources

• Jets in FRII sources (with the possible exception of the weakest examples) have _j 1 on all scales.

• They have << 1 and are highly supersonic.

• The post-shock flow is mildly relativistic in high-power sources (Laing 1989) but probably not in low-power ones (Black et al. 1992). This may require the shock to be oblique, consistent with the recessed appearance of many bright, compact hot-spots.

• v_h << v_j, estimates for v_h being in the range 0.02c - 0.2c.

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.

Acknowledgements

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.