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Buoyant bubbles of relativistic plasma in cluster cores plausibly play a keyrole in conveying the energy from a supermassive black hole to the intraclustermedium (ICM) - the process known as radio-mode AGN feedback. Energyconservation guarantees that a bubble loses most of its energy to the ICM aftercrossing several pressure scale heights. However, actual processes responsiblefor transferring the energy to the ICM are still being debated. One attractivepossibility is the excitation of internal waves, which are trapped in thecluster's core and eventually dissipate. Here we show that a sufficientcondition for efficient excitation of these waves in stratified clusteratmospheres is flattening of the bubbles in the radial direction. In ournumerical simulations, we model the bubbles phenomenologically as rigid bodiesbuoyantly rising in the stratified cluster atmosphere. We find that theterminal velocities of the flattened bubbles are small enough so that theFroude number ${\rm Fr}\lesssim 1$. The effects of stratification make thedominant contribution to the total drag force balancing the buoyancy force. Inparticular, clear signs of internal waves are seen in the simulations. Thesewaves propagate horizontally and downwards from the rising bubble, spreadingtheir energy over large volumes of the ICM. If our findings are scaled to theconditions of the Perseus cluster, the expected terminal velocity is$\sim100-200{\,\rm km\,s^{-1}}$ near the cluster cores, which is in broadagreement with direct measurements by the Hitomi satellite.

Generation of Internal Waves by Buoyant Bubbles in Galaxy Clusters and Heating of Intracluster Medium