  The time- and ensemble-averaged mechanical energy outputs of radio galaxies
may be large enough to offset much of the cooling inferred from X-ray
observations of galaxy clusters. But does this heating actually counterbalance
the cooling, diminishing cooling flows or quenching them altogether? I will
argue that energy injection by radio galaxies may be important even in clusters
where no active source is present, due to the likely intermittency of the jets.
If the energy injected by radio galaxies percolates through the intracluster
medium without excessive mixing, it could stabilize the atomic cooling
responsible for X-ray emission.
