The genesis of morphologies in extended radio sources: X-shapes, off-axis distortions and giant radio sources

Abstract

We examine relationships between the morphology in double radio sources and the radio-optical position angle offset - the relative orientation of the radio axis with respect to the major axis of the host galaxy. The study was done for a representative sample of radio sources, the nearby (redshift z < 0.5) 3CRR sources and separately for samples of giant radio sources and X-shaped radio sources. We find that radio morphological features have a dependence on the radio-optical position angle offset and on whether the source is a major- or minor-axis source. The evidence indicates an anisotropic gas environment, related to the ellipticity of the host galaxy, that causes the source linear size evolution, strength of backflow in the radio lobes, off-axis lobe distortions and the formation of wings and X-shaped radio sources to depend on the radio-optical position angle offset. We identify a class of X-shaped radio sources, which are either edge-darkened or lacking hotspots and appear to have inner doubles suggesting a restarting of activity. We suggest a common formation mechanism, requiring backflows, for these apparently FR-I X-shaped radio sources as well as the edge-brightened X-shaped sources.