The cosomological evolution of the environments of powerful radio   galaxies
  We present the results from the analysis of 26 extragalactic radio sources of type FRII which were observed with the VLA at 5 GHz and around the 1.4 GHz band. The sources were selected to have redshifts in the range $ 0.3<z<1.3$, radio powers between $6.9 \times 10^{26} {\rm WHz^{-1}}<P_{151 {\rm MHz}}<1.3 \times 10^{28} {\rm WHz^{-1}}$ and angular size $\theta \ge 10''$. We found that the depolarisation and the rms variations in the rotation measure increased with redshift. The flux values obtained from the observations were used to derive by means of analytical modelling the jet--power, density of the central environment, age of the source and its lobe pressure and the results were then compared with the observations. We find no significant correlations with the density parameter suggesting that the depolarisation and the rms variations in the rotation measure are indicative of the environment becoming more disordered rather than denser. The age and size of a source are correlated and both were found to be independent of redshift and radio--power. Jet--power strongly correlated with the radio--power. The lobe pressure was found to be anti--correlated with size which could explain why there are no sources beyond a few Mpc in size. We found no significant correlation between size and density which demonstrates that the sample is a fair representation of the population. 
J. A. Goodlet and C. R. Kaiser