The interpretation of asymmetric depolarization in extragalactic radio sources

Abstract

We discuss various interpretations of the effect found in powerful double radio sources with one-sided jets whereby the depolarization is systematically stronger on the counter-jet side. Depolarization by gas located within the source is unlikely to be the cause of the asymmetry. Line-emitting gas, though often associated with depolarization in radio galaxies, is not systematically stronger on the counter-jet side. We interpret the asymmetry in terms of a halo of hot gas surrounding the radio source. If the visible jet is on the near side of the source, as required for the relativistic beaming hypothesis, it will be seen through less depolarizing gas. We have investigated halo models, using density information from X-ray data and assuming the magnetic-field energy to scale as the gas density. Tangling of the magnetic field due to turbulence is shown to be important chiefly on scales of a few kpc. To produce the observed depolarization asymmetry the halo core diameter must be within a factor 2 of the source size, of order 100 kpc. Haloes of this size are associated with poor groups of galaxies. The correlation of depolarization asymmetry with redshift implies that the median halo size decreases by more than a factor of 2 at redshifts z > 1. Central values of n0B0 are about 1×10-3cm-3 μG at low redshift, increasing to 40 × 10-3 cm-3 μG for clusters at z > 1. For the most distant clusters, we estimate n0 ∼ 3.0 × 10-2 cm-3 and B0 ∼ 1 μG.