How AGN Jets Heat the Intracluster Medium -- Insights from Hydrodynamic
  Simulations
  Feedback from active galactic nuclei (AGN) is believed to prevent
catastrophic cooling in galaxy clusters. However, how the feedback energy is
transformed into heat, and how the AGN jets heat the intracluster medium (ICM)
isotropically, still remain elusive. In this work, we gain insights into the
relative importance of different heating mechanisms using three-dimensional
hydrodynamic simulations including cold gas accretion and momentum-driven jet
feedback, which are the most successful models to date in terms reproducing the
properties of cool cores. We find that there is net heating within two `jet
cones' (within ~30 degrees from the axis of jet precession) where the ICM gains
entropy by shock heating and mixing with the hot thermal gas within bubbles.
Outside the jet cones, the ambient gas is heated by weak shocks, but not enough
to overcome radiative cooling, therefore forming a `reduced' cooling flow.
Consequently, the cluster core is in a process of `gentle circulation' over
billions of years. Within the jet cones, there is significant adiabatic cooling
as the gas is uplifted by buoyantly rising bubbles; outside the cones, energy
is supplied by inflow of already-heated gas from the jet cones as well as
adiabatic compression as the gas moves toward the center. In other words, the
fluid dynamics self-adjusts such that it compensates and transports the heat
provided by the AGN, and hence no fine-tuning of the heating profile of any
process is necessary. Throughout the cluster evolution, turbulent energy is
only at the percent level compared to gas thermal energy, and thus turbulent
heating is not the main source of heating in our simulation.
