The Evolution of Radio Lobes in Galaxy Groups and Clusters

The synchrotron‐radiating particles and magnetic fields in low‐power radio galaxies (including the cluster‐center sources thought to regulate cooling flows), if at equipartition, can provide only a small fraction of the total internal energy of the radio lobes, which is now well‐constrained via X‐ray observations of the surrounding gas. I investigate the constraints on models for the dominant energy contribution in low‐power radio lobes via a detailed comparison of how the internal equipartition pressure and external pressure measured from X‐ray observations evolve with distance along the jet for two radio galaxies, 3C 31 and Hydra A. I conclude that models in which magnetic field or relativistic protons carried up the jet dominate lobe energetics are unlikely, and rule out electron dominance via inverse‐Compton constraints. Finally, I argue that entrainment of material from the jet surroundings can provide the necessary pressure, and show that a simple, physically motived model of the evolution of entrainment rate is consistent with the requirements for pressure balance along the 100‐kpc scale plumes of 3C 31.