The sidedness of jets and depolarization in powerful extragalactic radio sources

The cause of one-sidedness in the jets of otherwise symmetrical extragalactic radio sources is a subject of some controversy¹. There is good evidence for relativistic bulk motion in the cores of these objects (from observations of apparent superluminal motion, rapid variability and the absence of inverse-Compton X-rays)². A natural hypothesis is then that jets are intrinsically two-sided and that they remain relativistic on all scales³, the jet closer to us appearing bright as a result of Doppler beaming. Alternatively, the jets may be intrinsically one-sided, in which case they need not be relativistic. During a study of the polarization properties of powerful radio sources, it became clear that in those sources with one-sided jets, depolarization with increasing wavelength is usually weaker for the lobe containing the jet. One obvious interpretation is that the depolarization is caused by differential Faraday rotation through irregularities in a magnetoionic medium surrounding the radio source. The side with the stronger jet is closer to us, is seen through a smaller amount of material and therefore shows less depolarization. A halo of hot gas around the associated galaxy or quasar is a likely candidate for the depolarizing medium.