Misalignment and curvature effects in extragalactic radio sources

We have collected data for all extragalactic radio sources for which position angle information on both the pc and kpc scale is available. The difference of the position angle Delta PA has a maximum at small angles and a second peak at about 90 degrees as has been reported by several authors. We find that it appears generally difficult to fit the data with single population simple bend models, especially for intermediate redshift sources (0.35 < z < 1). We also find that the distribution of Delta PA for BL Lac objects and quasars is different at the 98% level. Allowing for a second population with intrinsically large bending and small Lorentz factors provides a significantly better description of the data. This is consistent with the properties of the misaligned sources, which have been inspected in detail. These objects form a rather inhomogeneous class and make very difficult an interpretation of the whole misaligned sample in terms of a single dominant mechanism. Individual cases may be described by helical trajectories due to precession in binary black hole systems. As an alternative to binary black holes we suggest the occurrence of twisted accretion discs in the central engines of some highly distorted radio sources. Several proper ties of misaligned source samples and secondary peaks in the distribution of misalignment angles can find simple explanations within such a scenario.