Thermal Balance in the Intracluster Medium: Is Agn Feedback Necessary?

A variety of physical heating mechanisms are combined with radiative cooling to explore, via one dimensional hydrodynamic simulations, the expected thermal properties of the intracluster medium (ICM) in the context of the cooling flow problem. Energy injection from type Ia supernovae, thermal conduction, and dynamical friction (DF) from orbiting satellite galaxies are considered as potential heating mechanisms of the central regions of the ICM, both separately and in conjunction. The novel feature of this work is the exploration of a wide range of efficiencies of each heating process. While DF and conduction can provide a substantial amount of energy, neither mechanism operating alone can produce nor maintain an ICM in thermal balance over cosmological timescales, in stark contrast with observations. For simulated clusters with initially isothermal temperature profiles, both mechanisms acting in combination result in long-term thermal balance for a range of ICM temperatures and for central electron densities less than ne ∼ 0.02 cm−3; at greater densities catastrophic cooling invariably occurs. Furthermore, these heating mechanisms can neither produce nor maintain clusters with a declining temperature profile in the central regions, implying that the observed “cooling-core” clusters, which have such declining temperature profiles, cannot be maintained with these mechanisms alone. Supernovae heating also fails to maintain clusters in thermal balance for cosmological timescales since such heating is largely unresponsive to the properties of the ICM. Thus, while there appears to be an abundant supply of energy capable of heating the ICM in clusters, it is extremely difficult for the energy deposition to occur in such a way that the ICM remains in thermal balance over cosmological time-scales. For intracluster media that are not in thermal balance, the addition of a small amount of relativistic pressure (provided by e.g. cosmic rays) forestalls neither catastrophic heating nor cooling. These conclusions are driven largely by the fact that 1) DF heating scales approximately as the gas density, while cooling scales as gas density squared, and thus DF heating cannot generically balance cooling without fine-tuning; 2) conduction acts to erase temperature gradients, while most observed clusters in fact show strong gradients in the inner regions. These results strongly suggest that a more dynamic heating process such as feedback from a central black hole is required to generate the properties of observed intracluster media. Subject headings: cooling flows — galaxies: clusters — hydrodynamics — conduction