The depolarisation properties of powerful radio sources : breaking the radio - power vs . redshift degeneracy

We define 3 samples of extragalactic radio sources of type FRII, containing 26 objects in total. The control sample consists of 6C and 7C sources with radio powers of around 10 27 W Hz −1 at 151 MHz and redshifts of z ∼ 1. The other samples contain 3CRR sources with either comparable redshifts but radio powers about a decade larger or with comparable radio powers but redshifts around z ∼ 0.4. We use these samples to investigate the possible evolution of their depolarisation and rotation measure properties with redshift and radio power independently. We used VLA data for all sources at ∼ 4800 MHz and two frequencies within the 1400 MHz band, either from our own observations or from the archive. We present maps of the total intensity flux, polarised flux, depolarisation, spectral index, rotation measure and magnetic field direction where not previously published. Radio cores were detected in twelve of the twenty-six radio sources. Fourteen of the sources show a strong Laing-Garrington effect but almost all of the sources show some depolarisation asymmetry. All sources show evidence for an external Faraday screen being responsible for the observed depolarisation. We find that sources at higher redshift are more strongly depolarised. Rotation measure shows no trend with either redshift or radio power, however variations in the rotation measure across individual sources increase with the redshift of the sources but do not depend on their radio power.