Central radio-emitting AGN strongly affect the X-ray-emitting gas in cooling flows. The radio sources create cavities or "bubbles" in the X-ray gas, which, in turn, confines the radio sources. In all clusters observed so far, there is no evidence that the radio sources are strongly shocking the ICM. The X-ray-bright shells surrounding the bubbles are cool, not hot. Weak shocks may have occurred in the past, creating the dense shells. The only evidence for strong-shock heating of a similar nature has been seen in radio-ISM interactions in galaxies, and there are only very few cases of this, so far.
The X-ray pressures derived from the shells surrounding the bubbles are approximately ten times higher than the radio equipartition pressures. There may be problems with some of the equipartition assumptions, or additional contributions to the pressure within the radio bubbles. One possibility for this additional pressure source is very hot, diffuse, thermal gas.
The bubble interiors are less dense than their surroundings and therefore will rise buoyantly outward into the clusters. Ghost cavities provide evidence that this process has occurred. The buoyantly-rising bubbles transport energy and magnetic fields into the clusters and they can also entrain cool gas from the cluster centers outward.
The shell pressures and bubble volumes measured in the X-ray can
be used to determine the total energy output of the radio sources. A
radio source repetition rate of
108 yr is
derived from the buoyancy rise time of the ghost cavities.
A rough comparison of the average energy output of radio sources and the
luminosity of cooling gas shows that the radio sources can supply enough
energy to offset the cooling in cooling flows, at least in some cases.
Acknowledgements
I am very grateful to my collaborators, Craig Sarazin, Brian McNamara, Noam Soker, Tracy Clarke, and Mike Wise. Support for E. L. B. was provided by NASA through the Chandra Fellowship Program, grant award number PF1-20017, under NASA contract number NAS8-39073.