The lobes of radio galaxies and quasars, fed by jets and hot spots, represent a significant, and currently ill-constrained, source of energy input into the intergalactic medium ( IGM). How much energy is input into the IGM depends on the minimum energy to which the power-law distribution of relativistic particles is accelerated in the hot spots. This has hitherto been unknown to within 3 orders of magnitude. We present direct evidence for the discovery of this low-energy cutoff in the lobe of a megaparsec-sized radio galaxy via the existence of extended X-ray emission, inverse Compton-scattered from aged radio plasma, and its separation by 80 kpc from regions containing freshly accelerated plasma from the hot spot. The low-energy cutoff of gamma similar to 10(4) in the hot spot is higher than previously thought but reconciles discrepancies with magnetic field estimates that had been systematically lower than equipartition values. The inverse Compton scattering of the spent synchrotron plasma is at the expense of cosmic microwave background (CMB) photons; we comment on the importance of such giant radio galaxies as contaminants of CMB anisotropies.
