Relic radio 'bubbles' and cluster cooling flows

Recent suggestions that buoyant radio emitting cavities in the intracluster medium (ICM) can cause significant reheating of cooling flows are re-examined when the effects of the intracluster magnetic field are included. Expansion of the cavity creates a tangential field in the ICM surrounding the cavity, and this field can suppress instabilities that mix the ICM and the radio source. Onset of instability can be delayed for similar to10(8) yr, and calculation of the subsequent turbulent cascade shows that actual reheating of the ICM may be delayed for up to similar to5 x 10(8) yr. These results may explain why the relic radio cavities remain as intact entities at times greater than or equal to10(8) yr, and the delay in injection of energy from the radio source into the ICM may mean that the role of radio sources in reheating cooling flows should be re-examined. In addition, the existence of relic radio cavities may also imply that the particle content of radio source lobes is primarily electrons and protons rather than electrons and positrons.