Depolarization in extragalactic radio sources is due to unresolved Faraday rotation fluctuations. That the rotation is due to foreground material is shown by high resolution observations which resolve the Faraday screen (Perley, Bridle & Willis 1984; Dreher et al. 1987). I have shown that the observed wavelength dependence of the polarization is consistent with a partially resolved foreground screen. The depolarization asymmetry is caused by a path length difference due to source orientation.

The strong correlation between the depolarization asymmetry and the spectral index asymmetry is compelling evidence that the latter is also an orientation effect that can be easily understood as a Doppler effect with the hotspots advancing at a few tenths of the speed of light. This adds further support to the relativistic beaming hypothesis for the observed jets on all scales.

The idea that the hotspots are advancing at such speeds is not new. This paper supports arguments put forward by Scheuer (1987) for large hotspot advance speeds and does not violate present constraints on the hotspot velocity. Longair & Riley (1979) derived an upper limit on the expansion velocity of 0.2c from the observed size asymmetry. Alexander & Leahy (1987) derived expansion velocities from spectral ageing which were a substantial fraction of 0.1c. Both these results give the mean advance rate rather than the instantaneous rate which could be higher if the working surface changes (Scheuer 1987).

The hotspot compactness asymmetry can be trivially explained as a result of beaming with a velocity gradient across the hotspot. This works well provided the emission is more extended than the velocity distribution. A natural consequence of this model is the possibility of an inverted receding hotspot where the central emission is reduced by beaming below the level of the surrounding slower emission. A spectral index gradient within the hotspot will give a spectral index asymmetry for the hotspots themselves.

The correlation between large- and small-scale sidedness rules out `flip-flop' models for the jets, as it shows that the jet direction has been constant for longer than the time required for the jet to travel from the nucleus to the hotspot.

Although I have argued that the observed asymmetries are simple orientation effects, this does not imply that the jets cannot be slightly asymmetric. However, observed structural asymmetries are rather modest, and very few sources have emission on one side of the nucleus only. It will be difficult to distinguish between asymmetric jets (Wang et al. 1991) and the effects of an asymmetric environment (McCarthy et al. 1991) as the overall results are rather similar. The correlation with the optical line emission is strong evidence that the environment is asymmetric and influencing the structure of radio sources. This makes a test for a jet power asymmetry problematic as there is no otherwise symmetric parent population. Another complicating factor is beaming, as this changes the apparent properties of the sources. Beaming and environment can both hide and mimic intrinsic asymmetries. The argument of Longair & Riley (1979) constraining the mean hotspot advance speeds to be less than 0.2c also implies that the mean advance speeds of the hotspots can only differ by ~20%. The effect of both intrinsic and environmental asymmetries will be to add noise to the depolarization and spectral index asymmetries.

The depolarization should depend on the arm-lengths of the two sides to a small extent because the depolarizing halo has a radial profile. This is only important for sources close to the plane of the sky where the effect of orientation is small. The lack of correlation between arm-length and depolarization for quasars implies that these sources are not close to the plane of the sky, whereas radio galaxies do show the effect, providing further independent support for unified schemes (Barthel 1989).

The high speeds inferred for parsec-scale radio jets extend all the way along the jets to the hotspots. This explains the flux asymmetry of the hotspots, the spectral index asymmetry, and the compactness of the hotspot on the jet side. With some scatter caused by arm-length and environment effects, the depolarization asymmetry is simply due to source orientation in a magnetized gaseous halo. The high speed of the hotspots leads to several other effects, such as inverted receding hotspots and spectral index asymmetries for the hotspots themselves, that should be seen as the quantity and quality of data improves.

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Peter Tribble,