  The hot plasma permeating clusters of galaxies often shows a central peak in
the X-ray surface brightness that is coincident with a drop in entropy. This is
taken as evidence for a cooling flow where the radiative cooling in the central
regions of a cluster causes a slow subsonic inflow. Searches in all wavebands
have revealed significantly less cool gas than predicted indicating that the
mass deposition rate of cooling flows is much lower than expected. However,
most cooling flow clusters host an Active Galactic Nucleus (AGN) at their
centres. AGN can inflate large bubbles of hot plasma that subsequently rise
through the cluster atmosphere, thus stirring this gas. Here we report on the
results from highly resolved hydrodynamic simulations which for the first time
show that buoyant bubbles increase the cooling time in the inner cluster
regions and thereby significantly reduce the deposition of cold gas. This work
demonstrates that the action of AGN in the centres of cooling flow clusters can
explain the observed absence of large quantities of cooled gas.
