Effects of Active Galaxy Feedback in Galaxy Clusters

Paul Ricker, University of Illinois at Urbana-Champaign

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The intracluster medium (ICM), the hot plasma trapped in the gravitational potential wells of galaxy clusters, exhibits a complex set of phenomena spanning more than seven orders of magnitude in length and time scales. The physical processes involved include dark matter-driven gravitational instability, multiphase magnetohydrodynamics (MHD) with possibly anisotropic conduction and viscosity, turbulence, radiative cooling, accretion onto black holes, relativistic jets, and relativistic particles mixed with the thermal plasma. These features make the ICM a natural setting for simulation studies that exploit the unique characteristics of Blue Waters.

We propose to apply a novel approach to a key problem impeding our understanding of the ICM: the feedback cycle by which relativistic jets produced by accreting black holes regulate the radiative cooling of the ICM. Observational evidence for such a feedback cycle is extremely strong. However, its physical explanation requires that we connect processes occurring in accretion disks around black holes smaller than the Solar System with plasma physics as much as 100 kpc¹ away. Direct simulation of this range of length scales with the full suite of necessary physics is beyond even Blue Waters.

The modeling of active galactic nucleus (AGN) feedback usually involves a sub-grid model and considerable simplification of the complex physics involved in the region surrounding the AGN’s central black hole. However, we have developed a new method that measures the accretion rate through an artificial control surface, incorporating the black hole plus accretion disk system and predicting the feedback efficiency self-consistently, reducing the number of free parameters compared to existing approaches.

With such models, we are able to perform simulations of the central kiloparsec scale region in the cores of galaxy clusters. By comparing these simulations with X-ray, optical, and radio observations we will be able to constrain the remaining physical parameters. With a better understanding of the physical processes involved, we will be able to develop AGN sub-grid models applicable to more coarsely refined (but larger volume) cosmological simulations performed using adaptive mesh refinement (AMR).  We will consider the roles played by time-varying feedback efficiency, multi-mode feedback, and shocks produced by galaxy cluster mergers.