Deep radio observations of 3C324 and 3C368: evidence for jet-cloud interactions

Abstract

High resolution, deep radio images are presented for two distant radio galaxies, 3C324 (z=1.206) and 3C368 (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 8 GHz, and using the MERLIN array at 1.4 and 1.65 GHz. Radio spectral index, radio polarisation, 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 3C368 shows a close correlation between the radio and optical emission, whilst along the jet direction of 3C324 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. 3C368 is strongly depolarised and has an average rest-frame rotation measure of a few hundred rad/m^2, reaching about 1000 rad/m^2 close to the most depolarised regions. 3C324 has weaker depolarisation, and an average rest-frame rotation measure of between 100 and 200 rad/m^2. Both sources show large gradients in their rotation measure structures, with variations of up to 1000 rad/m^2 over distances of about 10 kpc.