Depolarization and other asymmetries in double radio sources

Peter C. Tribble

Canadian Institute for Theoretical Astrophysics, 60 St. George Street, Toronto M5S 1A1, Canada

(Present address: Institute of Astronomy, Madingley Road, Cambridge CB3 0HA)

Accepted 1991 December 12. Received 1991 December 2; in original form 1991 August 20.


The presence of a one-sided jet in double radio sources is often interpreted as an orientation effect due to relativistic beaming. The counterjet side is more depolarized, consistent with a larger path length through a depolarizing medium. I consider this and other asymmetries in detail. The polarization curves of a small sample are shown to be consistent with the expected behaviour of a source behind a partially resolved foreground screen. I model the depolarization asymmetry and show how the orientation angles of the sources can be found. There is a deficit of sources in the plane of the sky for the Garrington et al. sample, although this may be a selection effect. Depolarization is only correlated with arm-length for sources close to the plane of the sky, such as radio galaxies. I show that the recently observed spectral index asymmetry can also be interpreted as an orientation effect if the hotspots advance at speeds of vapprox0.2c. This supports the idea that double sources are intrinsically two-sided and that the depolarization and spectral asymmetries are primarily due to orientation. High flow velocities in the hotspots give a natural explanation for the compactness asymmetry, and imply a spectral index asymmetry for the hotspots themselves.

Key words: polarization - galaxies: active - galaxies: jets - radio continuum: galaxies.


Models of the depolarization asymmetry
Fitting the depolarization data
Spectral index asymmetry
Hotspot asymmetries

This article appeared in Monthly Notices of the Royal Astronomical Society, vol 256, pp. 281-290 (1992). Copyright Royal Astronomical Society.
Peter Tribble,