WEAK HEADED QUASARS

We use the published VLA maps of high luminosity, steep-spectrum radio quasars showing prominent kiloparsec-scale one-sided jets, but faint hot-spots (i.e., weak-headed quasars) to call into question the common perception that the mechanism leading to such unusual morphologies is the strong dissipation of the jet's power through a vigorous entrainm ent of thermal gas by the initially relativistic jet. Under this hypot hesis nearly half of the weak-headed quasars would be predicted to exh ibit two-sided jets, one of which is actually receding from us, but is made visible nonetheless due to the postulated strong dissipation. Ho wever, no example of a source with two-sided jets is found in a well m apped representative sample of 8 weak-headed quasars. Evidently, the p rominent appearance of their (all one-sided) jets is still largely due to relativistic bulk flow. We therefore suggest that the anomalous we akness of the heads of the jets is probably linked to the weakening of the Mach disk, following the onset of decollimation of the jet's work ing surface as its forward motion slows down to nearly subsonic speed. We outline some potentially interesting observable consequences of th is scenario. Weak-headed quasars, which seem to be a substantial subse t of the steep-spectrum quasar population at large redshifts, could pr ovide important clues on the late evolutionary stages of powerful radi o sources.