1996A&A...316L..13G

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 entrainment of thermal gas by the initially relativistic jet. Under this hypothesis nearly half of the weak-headed quasars would be predicted to exhibit two-sided jets, one of which is actually receding from us, but is made visible nonetheless due to the postulated strong dissipation. However, no example of a source with two-sided jets is found in a well mapped representative sample of 8 weak-headed quasars. Evidently, the prominent appearance of their (all one-sided) jets is still largely due to relativistic bulk flow. We therefore suggest that the anomalous weakness 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 working surface as its forward motion slows down to nearly subsonic speed. We out-line some potentially interesting observable consequences of this scenario. Weak-headed quasars, which seem to be a substantial subset of the steep-spectrum quasar population at large redshifts, could provide important clues on the late evolutionary stages of powerful radio sources.