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

TL;DR

This study uses hydrodynamic simulations to evaluate if known heating mechanisms can maintain the thermal balance of the intracluster medium, concluding that additional dynamic processes like black hole feedback are likely necessary.

Contribution

It systematically explores the efficiencies of various heating mechanisms and demonstrates their insufficiency in sustaining thermal balance without black hole feedback.

Findings

Type Ia supernovae, conduction, and dynamical friction alone cannot maintain thermal balance.

These mechanisms fail to produce or sustain cooling-core temperature profiles.

A dynamic process like black hole feedback is likely essential for observed ICM properties.

Abstract

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. The novel feature of this work is the exploration of a wide range of efficiencies of each heating process. While the latter two 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 n_e~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. 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. 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. (ABRIDGED)