Abstract
We discuss two independent complete samples of radio sources for which extensive VLBI observations exist. The first sample is based on the Jodrell Bank 966 MHz survey and comprises 30 radio lobe dominated sources. Since radio lobe emission is accepted to be isotropic, this sample should be isotropically distributed with respect to the observer. And yet there seem to be more sources with apparent superluminal motion in their core than compatible with an isotropic distribution. The second sample is based on the Effelsberg 5 GHz survey north of declination 70° (S5) and comprises 13 flat spectrum radio sources. Here the evidence from VLBI, radio variability, optical polarization and X - r a y observations suggests that all sources in the sample show the effects of bulk relativistic motion in their nuclei. Combining the results from both samples leads to t h e suggestion that extended radio sources with strong heads like 3C179, 3C390.3 and S5 1928+73 maintain bulk relativistic motion all the way from the core to the strong emission knot in the head where multiple oblique shocks may dissipate the energy (Norman et al. 1984, Lind and Blandford 1986). The success of finding apparent superluminal motion in the 966 MHz sample is thus due to a selection effect which biases the sub-sample actually observed due to relativistic boosting mostly in the cores and to a small degree in the heads.
Dedicated to Prof. R. Kippenhahn on the occasion of his sixthieth birthday
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Biermann, P.L. (1987). Observations of a Complete Sample of Relativistic Sources in Active Galactic Nuclei. In: Hillebrandt, W., Meyer-Hofmeister, E., Thomas, HC. (eds) Physical Processes in Comets, Stars and Active Galaxies. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-72640-8_15
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