2.2. Radio Galaxies
At low redshifts, powerful radio sources are often associated with merger remnants; some 30% exhibit tails, fans, shells, or other signatures of recent collisions . But at redshifts z 0.6 the most striking morphological feature of powerful radio sources is a near-ubiquitous alignment between the radio lobes and continuum optical emission [25, 26]. This ``alignment effect'' seems at odds with the merger morphologies seen at low redshift; one explanation invokes jet-induced star formation (eg. ).
Recent observations suggest the alignment effect is compatible with mergers . Strong polarization is found in several z 2 radio galaxies, implying that the aligned emission is scattered light from an obscured AGN (eg. ); in several cases there is good evidence that dust is the primary scattering agent [29, 30]. HST imaging of the radio galaxy 0406-244 at z = 2.44 reveals a double nucleus and what appear to be tidal debris illuminated by an AGN .
From a theoretical perspective, merging may even be necessary to form powerful radio sources. The most plausible engines for such galaxies are rapidly spinning black holes (Blandford, these proceedings). Accretion from a disk can't spin up a black hole unless the accretion phase lasts ~ 0.1 Gyr; on the other hand, two black holes of comparable mass can coalesce to produce a rapidly-spinning hole .