``Speed bumps''

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4.1 ``Speed bumps''

One former stumbling-block to accepting large-scale energy transport at bulk-relativistic speeds got short shrift here. Theoretical beams emerge at right angles to small theoretical disks. The rough perpendicularity between kiloparsec-scale jets and kiloparsec-scale dust  ``lanes'' in a few radio galaxies (Kotanyi & Ekers 1979; Laing 1984) may have been given undue weight (given the small size of the sample, and the complexity of elliptical-galaxy dynamics) because of this. If such dust was used as a guide to jet orientation,  the jets often seemed too near to the sky plane for their (initial) sidedness -and in some sources the brighter jet was apparently receding. The radio depolarization  asymmetry has now supplanted the dust as a guide to jet orientations, both because the data are easier to come by and because they correlate so well with jet sidedness.  HST  images of the inner dust in radio galaxies (Jaffe et al. 1994; Lynds et al. 1994; S. A. Baum, private communication) also show that its distribution is not as simple as is needed for it to be a jet-orientation gauge. Benign neglect of the dust  (while exploring relativistic-jet models for all source powers) may be justified. But the dust should eventually fit into the picture-perhaps in the context of the dynamics of fueling the engine,  rather than of orienting its exhaust?

A second ``speed bump" could be the high incidence of counterjet candidates  in 3CR quasars, especially as these features are more prominent (relative to their lobes) than the jets in some FR II radio galaxies (Bridle et al. 1994). This result may be a problem if it persists in larger samples. If not, it may say that not every FR II radio galaxy ``hides'' an FR II quasar, as also suggested on spectroscopic grounds by Laing et al. (1994).

Alan Bridle
Wed Apr 10 10:19:46 EDT 1996