6 Conclusions

We have presented high resolution multi-frequency radio polarimetric observations of a sample of 27 high redshift radio galaxies. Maps of the sources and the fundamental parameters of the observations were presented. This, together with previous samples makes now an extended data base from which the relation between basic properties can be studied. The main results are the following:

• We detect radio cores in about half of the sample. The cores often have steep spectra ( $\alpha <-1$). The core fractions depend only weakly on radio sources size, contrary to the predictions of radio source evolutionary models. The median core fraction is larger than that of matched-luminosity 3CR radio galaxies at redshift $\sim 1$.
• We have shown that high redshift radio galaxies tend to be more distorted than at low redshift. This implies a larger density of the external medium in which they reside.
• We have discovered 8 new radio galaxies with very high Faraday rotation and large gradients between the different components. Given that the Faraday rotation properties do not depend on radio sources parameters such as power, total size, distortion etc., our interpretation is that these sources reside in very high density environments, possibly proto-clusters. We also find that the fraction of powerful radio galaxies with extreme Faraday rotation increases with redshift, in agreement with the change of their average environment with cosmic epoch.

Acknowledgements

The National Radio Astronomy Observatory is operated by Associated Univ. under contract with the National Science Foundation. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.