Evidence that low-redshift quasars frequently occur in interacting systems has been accumulating for two decades . Early claims that quasars have close companions are supported by recent studies out to redshifts z ~ 1 [32, 33, 34]. Even more telling are the tidal tails and other signs of violent interactions in nearby cases [35, 36, 37, 38, 39].
The very nature of these interactions makes their detection difficult at higher redshifts - tidal tails and other signs are hidden by cosmological dimming and quasar glare. Nor does the low-z evidence preclude the possibility that high-redshift quasars may have nothing to do with mergers. However, the peak in quasar activity at z ~ 2 to 3 seems to broadly coincide with other indications of extensive merging activity reviewed above. Given the observational difficulties, a compelling case that this high-z activity is driven by mergers probably awaits a theory for the formation of supermassive black holes.