A redshift dependence of the maximum distance out to which active galactic nuclei (AGN) can transport their relativistic particles was first found for radio quasars by Miley1. He demonstrated that this distance was, on average, smaller in the past than it is at present, Analysing the run of median angular size with flux density for the entire extragalactic radio source population, Kapahi2 concluded that the radio galaxy population probably shares this property with the extended radio quasars. By using samples of radio ellipticals complete to vastly different flux limits, we can, for the first time, separate redshift- and luminosity-dependences of their linear sizes. Ram pressure in a medium with cosmologically varying density will naturally cause a dependence on redshift3, but this cannot explain so steep a dependence as that implied here. It may be expected if galaxy haloes influence the outward energy transport4.
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