Heating, conduction, and minimum temperatures in cooling flows

There is mounting observational evidence from Chandra for strong interaction between keV gas and active galactic nuclei (AGNs) in cooling flows. It is now widely accepted that the temperatures of cluster cores are maintained at a level of 1 keV and that the mass deposition rates are lower than earlier ROSAT/Einstein values. Recent theoretical results suggest that thermal conduction can be very efficient even in magnetized plasmas. Motivated by these discoveries, we consider a double heating model that incorporates the effects of simultaneous heating by both the central AGN and thermal conduction from the hot outer layers of clusters. Using hydrodynamic simulations, we demonstrate that there exists a family of solutions that does not suffer from the cooling catastrophe. In these cases, clusters relax to a stable final state, which is characterized by minimum temperatures of order 1 keV and density and temperature profiles consistent with observations. Moreover, the accretion rates are much reduced, thereby reducing the need for the excessive mass deposition rates required by the standard cooling flow models.