2000MNRAS.317..120R

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=50km.s^–1.Mpc^–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 <12° alignments appear to be produced by merging companion galaxies at position angles close to the radio axes. In five galaxies, the radio axes and the projected position angles of the interacting pairs coincide within 21°, and there is evidence that the radio jet interacts with the companion. As we estimate that only one such alignment would occur by chance in our sample, this may be evidence for a second type of alignment effect produced by companion galaxies. For the cluster cD galaxy 3C 348, we find some evidence that the distribution of cluster galaxies is aligned with the radio axis.

Most (10 out of 15) of the radio galaxies have optical half-light radii consistent with the size-luminosity relation of local radio-quiet early-type galaxies, the exceptions being three known cluster cDs, which have larger radii than normal ellipticals, and the two galaxies at the highest redshifts, which are ∼0.8mag more luminous than expected for their size. Most (13 out of 15) have radial profiles best fitted by a bulge [exp(-rr_e)^0.25] model, but two are closer in a χ² test to an exponential profile. One of these, 3C 285, appears sufficiently well-imaged to be classified as a disc galaxy and shows evidence of a previous merger on a polar axis.