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 8 GHz, and using the MERLIN array at 1.4 and 1.65 GHz. 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 rad m−2, reaching about 1000 rad m−2 close to the most depolarized regions. 3C 324 has weaker depolarization, 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.
Highlights
The discovery that the optical, ultraviolet and line emission of powerful high-redshift radio galaxies is elongated and aligned along the direction of the radio axis (McCarthy et al 1987; Chambers et al 1987), a phenomenon known as the ‘alignment effect’, indicates a close association between radio source activity and the optical emission of these galaxies
(ii) The radio cores of each source lie at a minimum of the optical emission, probably associated with obscuration by a central dust lane
(iii) Radio jets are detected in both sources, and in each case are co-linear with the strings of bright optical knots. (iv) These observations sort out the disparity between lower angular resolution observations of the polarization structures of these radio galaxies (Pedelty et al 1989; Fernini et al 1993)
Summary
The discovery that the optical, ultraviolet and line emission of powerful high-redshift radio galaxies is elongated and aligned along the direction of the radio axis (McCarthy et al 1987; Chambers et al 1987), a phenomenon known as the ‘alignment effect’, indicates a close association between radio source activity and the optical emission of these galaxies. High-resolution radio observations of these sources provide a critical test of the interaction / illumination models: if the locations of the radio jets and the strings of optical activity are co–linear, the influence of the radio jets must play a key role in the alignment effect This would not, provide direct proof of the jet-induced star formation hypothesis: the emission may be nebular continuum emission from warm gas shocked by the radio jets, or may be associated with an increased scattering efficiency in these regions, for example by jet shocks breaking up cold gas clouds and exposing previously hidden dust grains, thereby increasing the surface area for scattering along the jet axis (Bremer, Fabian & Crawford 1997). Throughout the paper we assume H0 1⁄4 50 km s11 Mpc and Q 1⁄4 1, and all positions are given in equinox J2000 coordinates
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