The environments of FRII radio sources

Using ROSAT observations, we estimate gas pressures in the X-ray-emitting m edium surrounding 63 FRII radio galaxies and quasars. We compare these pres sures with the internal pressures of the radio-emitting plasma estimated by assuming minimum energy or equipartition. In the majority of cases (includ ing 12/13 sources with modelled, spatially resolved X-ray emission) radio s ources appear to be underpressured with respect to the external medium, sug gesting that simple minimum-energy arguments underestimate the internal ene rgy density of the sources. We discuss possible departures from the minimum -energy condition and the consequences of our result for models of the dyna mics of radio galaxies, in particular self-similar models.