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Intergalactic Matter and Cocoons of Radio Galaxies

The cocoons surrounding powerful radio sources can be extensive if the jet that feeds the cocoon is light and supersonic. They have been shown to remain overpressured with respect to the ambient medium for most of the life time of the sources. The observed lobes of the radio sources form parts of these extensive cocoons. We show that observations of the lobes of giant radio sources allow one to estimate the {\it density} of the intergalactic medium (IGM) in which the lobes are embedded. We estimate the IGM density to be of the order of a few percent of the closure density of the universe. We further calculate the radio power of the overpressured cocoon as a function of time and the ambient density.