Galaxy Cluster Astrophysics and Cosmology: Questions and Opportunities for the Coming Decade

Galaxy Cluster Astrophysics and Cosmology: Questions and Opportunities for the Coming Decade 1 Motivation and questions Clusters of galaxies provide us the opportunity to study an "ecosystem" -a volume that is a high-density microcosm of the rest of the Universe. Clusters are signposts for early structure formation, and are moderately isolated, growing on the Hubble timescale from the Cosmic Web. Clusters are excellent laboratories for studying plasma physical processes as well as for studying how super-massive black holes interact with the ambient cluster plasma. The next generation of cluster surveys is well suited to address fundamental problems in physics and cosmology (e.g. [1]), such as further constraining the Dark Energy equation of state [2] or to test whether our understanding of gravity is complete. Time is ripe to tackle the following important questions with clusters on an individual basis or as an entire population using multi-wavelength observational campaigns:

• How do clusters form and grow? How does feedback from star, galaxy, and black hole formation impact cluster structure and evolution? What detailed physical processes govern the heating and cooling of cluster cores? Are clusters pre-heated before they are assembled? What are the robust mass observables and scaling relations that can be used for cosmology? • How do the cluster medium and its constituents evolve? How much pressure is provided by the thermal plasma, by turbulence, and by the non-thermal particle populations? How are metals mixed into the medium? How does the hot ICM affect evolution of cluster galaxies and vice versa? • How do we use clusters of galaxies as a window on fundamental physics? Does Dark Matter interact or is it collisionless? Does Dark Matter annihilate? How does Dark Energy affect the growth and evolution of clusters? Does Dark Energy only change the expansion history of the Universe or is our understanding of gravity on the largest scales of the Universe incomplete? Do exotica (cosmic strings) impact cluster and structure formation in any way? • What can we learn about plasma astrophysics in clusters? What causes nonthermal high energy cluster emission in the radio and hard X-rays? What is the origin of large scale magnetic fields and how do they evolve? How do shocks of moderate strength accelerate relativistic particles? What are the properties of turbulence in the cluster medium, and how is this coupled to the cluster structure? Do anisotropic transport processes in the collisionless plasma play an important role? These questions are closely interlinked and require a multifaceted approach bridging the observational and theoretical. Complementary to other approaches to measuring the expansion history of the Universe (e.g. SNeIa, BAO), future large surveys of galaxy clusters potentially provide a powerful probe for the growth of structure and hence are an invaluable tool for testing our understanding of General Relativity. This however is only possible if systematics associated with cluster mass calibrations, sample selection effects, and our Bottom row L to R: Associated observables X-ray surface brightness, amplitude of the SZ effect, and the radio synchrotron emission. Note that each of these observables is sensitive to different physical properties and combining different observables enables us to solve for the underlying physics. Taken from [3]. incomplete knowledge of the physics of the intracluster medium can be understood and controlled.

Clusters of galaxies provide unique opportunities for investigating non-equilibrium processes in plasma astrophysics. Combining cluster observables from the radio to Gamma-rays enables us to probe unique characteristics of the intracluster plasma. Conditions range from magnetically dominated regions within radio lobes to plasma dominated by thermal pressure, with bulk motions being subsonic within cooling cores to predominantly supersonic in the accretion regions. Hence, observationally and theoretically, we expect a complex interplay between cosmic rays, magnetic fields, and turbulence which poses a range of intriguing physical puzzles. There are many excellent recent papers on observational and theoretical aspects of cluster astrophysics and cosmology, e.g. [4]. 1

In the past decade, great advances have been made in our understanding of clusters of galaxies, from the standpoint of their internal structure and evolution to their place in the larger scale structure of the Universe. These advances have been made possible by tremendous improvements in theoretical modeling and numerical simulation, as well as a wealth of new information provided by multi-wavelength surveys of the Universe. Below is a list of highlights that are jumping off points for future work: Explosion of Cluster Observations: SDSS and other optical/IR galaxy surveys have yielded many new candidates. The ROSAT cluster surveys are still our best all-sky X-ray sample, and Chandra and XMM-Newton

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