Reversing cooling flows with AGN jets: shock waves, rarefaction waves,
  and trailing outflows
  The cooling flow problem is one of the central problems in galaxy clusters,
and active galactic nucleus (AGN) feedback is considered to play a key role in
offsetting cooling. However, how AGN jets heat and suppress cooling flows
remains highly debated. Using an idealized simulation of a cool-core cluster,
we study the development of central cooling catastrophe and how a subsequent
powerful AGN jet event averts cooling flows, with a focus on complex
gasdynamical processes involved. We find that the jet drives a bow shock, which
reverses cooling inflows and overheats inner cool core regions. The shocked gas
moves outward in a rarefaction wave, which rarefies the dense core and
adiabatically transports a significant fraction of heated energy to outer
regions. As the rarefaction wave propagates away, inflows resume in the cluster
core, but a trailing outflow is uplifted by the AGN bubble, preventing gas
accumulation and catastrophic cooling in central regions. Inflows and trailing
outflows constitute meridional circulations in the cluster core. At later
times, trailing outflows fall back to the cluster centre, triggering central
cooling catastrophe and potentially a new generation of AGN feedback. We thus
envisage a picture of cool cluster cores going through cycles of
cooling-induced contraction and AGN-induced expansion. This picture naturally
predicts an anti-correlation between the gas fraction (or X-ray luminosity) of
cool cores and the central gas entropy, which may be tested by X-ray
observations.
