Generation of Internal Waves by Buoyant Bubbles in Galaxy Clusters and
  Heating of Intracluster Medium
  Buoyant bubbles of relativistic plasma in cluster cores plausibly play a key
role in conveying the energy from a supermassive black hole to the intracluster
medium (ICM) - the process known as radio-mode AGN feedback. Energy
conservation guarantees that a bubble loses most of its energy to the ICM after
crossing several pressure scale heights. However, actual processes responsible
for transferring the energy to the ICM are still being debated. One attractive
possibility is the excitation of internal waves, which are trapped in the
cluster's core and eventually dissipate. Here we show that a sufficient
condition for efficient excitation of these waves in stratified cluster
atmospheres is flattening of the bubbles in the radial direction. In our
numerical simulations, we model the bubbles phenomenologically as rigid bodies
buoyantly rising in the stratified cluster atmosphere. We find that the
terminal velocities of the flattened bubbles are small enough so that the
Froude number ${\rm Fr}\lesssim 1$. The effects of stratification make the
dominant contribution to the total drag force balancing the buoyancy force. In
particular, clear signs of internal waves are seen in the simulations. These
waves propagate horizontally and downwards from the rising bubble, spreading
their energy over large volumes of the ICM. If our findings are scaled to the
conditions of the Perseus cluster, the expected terminal velocity is
$\sim100-200{\,\rm km\,s^{-1}}$ near the cluster cores, which is in broad
agreement with direct measurements by the Hitomi satellite.
