Annu. Rev. Astron. Astrophys. 1984. 22: 319-58 Copyright © 1984 by Annual Reviews. All rights reserved

7. SUMMARY AND SOME KEY EXPERIMENTS

Jets occur often, in a wide range of extragalactic sources, and with properties well correlated with those of the compact radio cores; thus, it is reasonable to relate them to the fundamental process of energy transport from the cores to the lobes. Their presence supports continuous flow source models and shows that collimation, particle acceleration, and magnetic field amplification probably all occur on both parsec and kiloparsec scales in extragalactic sources. Beyond this, knowledge of jet physics is fragmentary, mainly because we lack credible estimates of jet densities and have only loose, model-dependent constraints on their velocities. Some important questions may be answerable, however, by observations with present or planned instruments:

1. How well does sidedness on parsec and kiloparsec scales correlate with core superluminal motion? Does superluminal motion occur in the cores of sources that should be oriented toward the plane of the sky (e.g. very large lobe-dominated sources)? (Both require sensitive, high dynamic range phase-closure VLBI mapping of cores that are not selected for high flux density alone.)

2. How asymmetric are one-sided kiloparsec-scale jets? Mild brightness asymmetries , compatible with weak Doppler boosting (Section 6.1.7) or small differences in the ratio of radiative losses to bulk energy flux, are much easier to explain in large samples than 100 : 1 (Sections 6.1.7 and 6.5). (This requires high dynamic range maps of large jets whose cores are not too dominant.)

3. Are jets brighter relative to the lobes when the core is also brighter? Does the answer vary with FR class or optical identification? (This requires unbiased statistics of core, jet, and lobe powers for identified sources.)

4. Can studies of the lobes distinguish the Doppler boosting, asymmetric dissipation, or "flip-flop" models of jet sidedness (Sections 6.2 and 6.3)? (This requires studies of the shapes, spectra, and degrees of polarization of hot spots in jetted and unjetted lobes.)

5. Are jets confined thermally or magnetically (Section 4.1)? Thermal confinement can be tested by high-resolution X-ray imaging and temperature determinations of the environs of recollimating jets, and magnetic confinement may be checked by radio polarimetry of jet cocoons (Section 4.1.3).

6. Can sharp brightness gradients in knots in kiloparsec-scale jets be used to constrain models for jet one sidedness (80)? (This requires proper motion studies of knots in nearby kiloparsec-scale jets.)

7. Do any jets unambiguously show depolarization that cannot be attributed to foreground Faraday screens, and that might therefore be used to indicate jet densities (Section 6.1)?

Finally, radio, optical, or X-ray spectroscopic evidence for outflow in jets will be welcome now that jets are being interpreted as tracers of the paths of energy transfer in all extragalactic sources.

ACKNOWLEDGMENTS

We are indebted to many colleagues who sent us unpublished data on jets, and who are credited individually in the footnotes to Table 1. We also thank Robert Laing, Bob Sanders, and Dick Henriksen for many invigorating discussions, and Peter Scheuer for valuable criticism of an early draft of this review.