  Using a self-similar model for the expansion of cocoons surrounding the jets
in powerful extragalactic radio sources (type FRII), we investigate the
influence of the properties of the gas surrounding these objects on their
evolution. The variation of their radio luminosity as a function of linear size
for individual sources is determined in dependence of the density distribution
of the IGM the source is embedded in. Based on these results, the cosmological
evolution of the FRII population and hence that of the environments of these
sources can be constrained. The environments are found to be denser and/or more
extended at high redshifts. The bow shock surrounding the cocoon of FRII
sources compresses and heats the IGM. A numerical integration of the
hydrodynamic equations governing the gas flow between the bow shock and the
cocoon boundary is presented. From this we determine the appearance of the
large scale structure of radio galaxies in X-rays and the cooling times for the
IGM heated by the passage of the bow shock. This has important implications for
the cosmological evolution of the IGM.
