MISALIGNMENT AND CURVATURE EFFECTS IN EXTRAGALACTIC RADIO-SOURCES

We have collected data for all extragalactic radio sources for which p osition angle information on both the pc and kpc scale is available. T he difference of the position angle Delta PA has a maximum at small an gles and a second peak at about 90 degrees as has been reported by sev eral authors. We find that it appears generally difficult to fit the d ata with single population simple bend models, especially for intermed iate redshift sources (0.35 < z < 1). We also find that the distributi on of Delta PA for BL Lac objects and quasars is different at the 98% level. Allowing for a second population with intrinsically large bendi ng and small Lorentz factors provides a significantly better descripti on of the data. This is consistent with the properties of the misalign ed sources, which have been inspected in detail. These objects form a rather inhomogeneous class and make very difficult an interpretation o f the whole misaligned sample in terms of a single dominant mechanism. Individual cases may be described by helical trajectories due to prec ession in binary black hole systems. As an alternative to binary black holes we suggest the occurrence of twisted accretion discs in the cen tral engines of some highly distorted radio sources. Several proper ti es of misaligned source samples and secondary peaks in the distributio n of misalignment angles can find simple explanations within such a sc enario.