Deep radio observations of 3C 324 and 3C 368: evidence for jet-cloud interactions

Abstract

High-resolution, deep radio images are presented for two distant radio galaxies, 3C 324 (z=1.206) and 3C 368 (z=1.132), which are both prime examples of the radio-optical alignment effect seen in powerful radio galaxies with redshifts z>~0.6. Radio observations were made using the Very Large Array in A-array configuration at 5 and 8GHz, and using the MERLIN array at 1.4 and 1.65GHz. Radio spectral index, radio polarization, and rotation measure maps are presented for both sources. Radio core candidates are detected in each source, and by aligning these with the centroid of the infrared emission the radio and the optical/infrared images can be related astrometrically with 0.1 arcsec accuracy. In each source the radio core is located at a minimum of the optical emission, probably associated with a central dust lane. Both sources also exhibit radio jets which lie along the directions of the bright strings of optical knots seen in high-resolution Hubble Space Telescope images. The northern arm of 3C 368 shows a close correlation between the radio and optical emission, whilst along the jet direction of 3C 324 the bright radio and optical knots are co-linear but not co-spatial. These indicate that interactions between the radio jet and its environment play a key role in producing the excess ultraviolet emission of these sources, but that the detailed mechanisms vary from source to source. 3C 368 is strongly depolarized and has an average rest-frame rotation measure of a few hundred radm^-2, reaching about 1000radm^-2 close to the most depolarized regions. 3C 324 has weaker depolarization, and an average rest-frame rotation measure of between 100 and 200radm^-2. Both sources show large gradients in their rotation measure structures, with variations of up to 1000radm^-2 over distances of about 10kpc.