Giant radio galaxies and cosmic-ray acceleration

Giant radio galaxies (GRGs) are prime and unique laboratories for constraining the plasma processes that accelerate relativistic electrons within large intergalactic volumes. The evidence for short radiative loss times rules out certain scenarios for energy transport within their very large dimensions. This, combined with their high energy content, large ordered magnetic field structures, the absence of strong large-scale shocks, and very low upper limits on their internal thermal plasma densities, points to a direct and efficient conversion of force-free magnetic field to particle energy. This is underlined by the evidence in GRGs that their internal Alfven speeds are higher than the lobe expansion speeds. We discuss these constraints in the context of models in which the central black hole energy is initially extracted as electromagnetic Poynting flux that injects large amounts of magnetic flux into the lobes. Recent advances in the theory of collisionless magnetic reconnection make this a favored mechanism to explain the particle acceleration in these systems. The energy reservoir is likely to be force-free fields, which is independently consistent with recent models of initial electromagnetic energy transfer from the parent galaxy's supermassive black hole. Such a scenario has wide-ranging astrophysical consequences: it implies that space-distributed magnetic reconnection or some other highly efficient field-to-particle energy conversion process likely dominates in all extended extragalactic radio sources.