Is cosmic ray heating relevant in cool core clusters?

Abstract

The absence of large cooling flows in cool core clusters appears to require self-regulated energy feedback by active galactic nuclei (AGNs) but the exact heating mechanism has not yet been identified. Here, we analyse whether a combination of cosmic-ray (CR) heating that is provided by the AGN and thermal conduction can offset radiative cooling. Using a large sample of about 40 cool core clusters, we determine steady state solutions of the hydrodynamic equations that are coupled to an evolution equation for the CRs. We find stable solutions that match the observed density and temperature profiles for all our clusters well. Radiative cooling is balanced by CR heating in the cluster centres and by thermal conduction on larger scales, thus demonstrating the relevance of both heating mechanisms. Our mass deposition rates vary by three orders of magnitude and are linearly correlated to the observed star formation rates. Clusters with large mass deposition rates show larger cooling radii and require a larger radial extent of the CR injection function. However, the resulting radio-synchrotron and gamma-ray emission of clusters that are hosting radio mini halos is in conflict with observational data suggesting that these clusters cannot be stably heated by CRs. Most interestingly, this inability of supplying sufficient heat to the cooling cluster gas is reflected in the thermodynamic profiles of these cool cores: clusters with radio mini halos are characterised by the largest cooling radii, star formation and mass deposition rates in our sample and thus signal the presence of a higher cooling activity. On the contrary, clusters without radio mini halos show little signs of cooling and appear to be stably heated. This diversity of cool cores for the first time demonstrates evidence for a heating-cooling cycle in cool cores with potentially CR heating from AGNs as the underlying physical source of heat. Contributors:C.Pfrommer, S. Jacob (HITS Heidelberg)C. Pfrommer, HITS Heidelberg, christoph.pfrommer@h-its.org S. Jacob, HITS Heidelberg, svenja.jacob@h-its.org