  The synchrotron-radiating particles and magnetic fields in low-power radio
galaxies (including most nearby cluster-centre sources), if at equipartition,
can provide only a small fraction of the total internal energy density of the
radio lobes or plumes, which is now well constrained via X-ray observations of
their external environments. We consider the constraints on models for the
dominant energy contribution in low-power radio-galaxy lobes obtained from a
detailed comparison of how the internal equipartition pressure and external
pressure measured from X-ray observations evolve with distance for two radio
galaxies, 3C 31 and Hydra A. We rule out relativistic-lepton dominance of the
radio lobes, and conclude that models in which magnetic field or relativistic
protons/ions carried up the jet dominate lobe energetics are unlikely. Finally,
we argue that entrainment of material from the jet surroundings can provide the
necessary pressure, and construct a simple self-consistent model of the
evolution of the entrainment rate required for pressure balance along the
100-kpc scale plumes of 3C 31. Such a model requires that the entrained
material is heated to temperatures substantially above that of the surrounding
intra-group or cluster medium, and that the temperature of the thermal
component of the jet increases with distance, though remaining
sub-relativistic.
