Both active and starburst galactic nuclei are best understood if they are fuelled by gas inflow to the nucleus (e.g. Simkin, Su & Schwarz 1980; Hutchings & Campbell 1983).
There is observational evidence that the occurrence of Seyfert and QSO activity is enhanced in the presence of nearby companions (Dahari 1984; Keel et al 1985; Hummel et al. 1987). At least some samples of Seyfert host galaxies show more close companions than would be expected for a random sample of similar galaxies, though the situation for tidal disturbances themselves is more complex. The selection effects modelled by Byrd et al. (1987) suggest that virtually all the Seyfert galaxies in Dahari’s sample have a companion which is (in principle) physically sufficient to provoke gas inflow in the disk.