Cause and Effect of Feedback: Multiphase Gas in Cluster Cores Heated by
  AGN Jets
  Multiwavelength data indicate that the X-ray emitting plasma in the cores of
galaxy clusters is not cooling catastrophically. To large extent, cooling is
offset by heating due to active galactic nuclei (AGN) via jets. The cool-core
clusters, with cooler/denser plasmas, show multiphase gas and signs of some
cooling in their cores. These observations suggest that the cool core is
locally thermally unstable while maintaining global thermal equilibrium. Using
high-resolution, three-dimensional simulations we study the formation of
multiphase gas in cluster cores heated by highly-collimated bipolar AGN jets.
Our key conclusion is that spatially extended multiphase filaments form only
when the instantaneous ratio of the thermal instability and free-fall
timescales (t_TI/t_ff) falls below a critical threshold of \approx 10. When
this happens, dense cold gas decouples from the hot ICM phase and generates
inhomogeneous and spatially extended Halpha filaments. These cold gas clumps
and filaments `rain' down onto the central regions of the core, forming a cold
rotating torus and in part feeding the supermassive black hole. Consequently,
the self-regulated feedback enhances AGN heating and the core returns to a
higher entropy level with t_TI/t_ff > 10. Eventually the core reaches
quasi-stable global thermal equilibrium, and cold filaments condense out of the
hot ICM whenever t_TI/t_ff \lesssim 10. This occurs despite the fact that the
energy from AGN jets is supplied to the core in a highly anisotropic fashion.
The effective spatial redistribution of heat is enabled in part by the
turbulent motions in the wake of freely-falling cold filaments. Increased AGN
activity can locally reverse the cold gas flow, launching cold filamentary gas
away from the cluster center. Our criterion for the condensation of spatially
extended cold gas is in agreement with observations and previous idealized
simulations.
