  It is generally argued that most clusters of galaxies host cooling flows in
which radiative cooling in the centre causes a slow inflow. However, recent
observations by Chandra and XMM conflict with the predicted cooling flow rates.
Here we report highly resolved hydrodynamic simulations which show that buoyant
bubbles can offset the cooling in the inner regions of clusters and can
significantly delay the deposition of cold gas. The subsonic rise of bubbles
uplifts colder material from the central regions of the cluster. This colder
material appears as bright rims around the bubbles. The bubbles themselves
appear as depressions in the X-ray surface brightness as observed in a growing
number of clusters.
