Optical/ultraviolet morphology and alignment of low-redshift radio galaxies

Fifteen of the lower redshift (0.0<z<0.6) 3CR radio galaxies were observed with the Wide Field Camera on the Isaac Newton Telescope, in the U band and in either the V or R bands. We find that at least 13 out of 15 of these galaxies are visibly interacting with neighbours or merging, providing strong evidence that their radio bursts are triggered by interactions. The mean projected separation of the interacting pairs is 44.0±9.6kpc (for H0=50kms−1Mpc−1), which is consistent with the mean pair separation of interacting Fanaroff-Riley Class I radio galaxies but approximately three times greater than that of interacting ultraluminous infrared galaxies. This implies that radio bursts typically occur at a much earlier stage of an interaction than the peak of a far-infrared luminous starburst. In six out of 15 radio galaxies, the optical structure is closely (within 12°) aligned with the radio axis. Three of these, which are the most radio-luminous galaxies in our sample, show an alignment of their optical long axis. The other three