Extended X-ray emission at high redshifts: radio galaxies versus clusters

Most old, distant radio galaxies should be extended X-ray sources because of inverse Compton scattering of cosmic microwave background (CMB) photons. Such sources can be an important component in X-ray surveys for high-redshift clusters, because of the increase with redshift of both the CMB energy density and the radio source number density. We estimate a lower limit to the space density of such sources and show that inverse Compton scattered emission may dominate above redshifts of 1 and X-ray luminosities of 10⁴⁴ergs^-1, with a space density of radio galaxies >10^-8Mpc^-3. The X-ray sources may last longer than the radio emission and so need not be associated with what is seen to be a currently active radio galaxy.