Interactions are evident in the more powerful objects only. The FR-I objects have low-power, radio jets declining with distance, and are in general elliptical galaxies. Fewer than 10% show tails, and tidal interactions (Smith & Heckman 1989). The rarer FR-II objects, with high-power, are classical doubles (Cygnus A, Perseus A). They show a high percentage of interactions, from 32% (Yates et al 1989) to 100% (Hutchings 1987); most of them (> 50%) have at least 2 companions, blue colors, star-forming regions (Heckman 1990, Gubanov 1991). Against the central radio-source, HI is seen in absorption, revealing that gas is most of the time infalling (absorption lines are blue-shifted, van Gorkom et al 1989).
Radio-loud QSO have 4-5 times more neighbors (Yee & Green 1984, Smith & Heckman 1990), while radio-quiet have 2 times more neighbors only with respect to a control sample. The morphology of QSO hosts is disturbed for 35-55% of them (Hutchings et al 1984, Smith et al 1986), while radio-loud are perturbed at 70-80% (Smith et al 1986, Hutching 1987); this is confirmed with HST (Disney et al 1995).
The radio power of radio-galaxies has been related to the spin-down of a rotating supermassive black hole (Begelman 1986). The spin of the black hole may be acquired in galaxy interactions and mergers: either the black hole is spin up through gas accretion from an external disk, or through the merging of two black holes. In this respect, it is significant that radio-galaxies are generally giant ellipticals, since these are expected to be formed essentially by mergers.