Cosmic Rays in Galaxy Clusters and Their Nonthermal Emission

Abstract

Radio observations prove the existence of relativistic particles and magnetic field associated with the intra-cluster-medium (ICM) through the presence of extended synchrotron emission in the form of radio halos and peripheral relics. This observational evidence has fundamental implications on the physics of the ICM. Non-thermal components in galaxy clusters are indeed unique probes of very energetic processes operating within clusters that drain gravitational and electromagnetic energy into cosmic rays and magnetic fields. These components strongly affect the (micro-)physical properties of the ICM, including viscosity and electrical conductivities, and have also potential consequences on the evolution of clusters themselves. The nature and properties of cosmic rays in galaxy clusters, including the origin of the observed radio emission on cluster-scales, have triggered an active theoretical debate in the last decade. Only recently we can start addressing some of the most important questions in this field, thanks to recent observational advances, both in the radio and at high energies. The properties of cosmic rays and of cluster non-thermal emissions depend on the dynamical state of the ICM, the efficiency of particle acceleration mechanisms in the ICM and on the dynamics of these cosmic rays. In this review we discuss in some detail the acceleration and transport of cosmic rays in galaxy clusters and the most relevant observational milestones that have provided important steps on our understanding of this physics. Finally, looking forward to the possibilities from new generations of observational tools, we focus on what appear to be the most important prospects for the near future from radio and high-energy observations.

Turbulence and vorticity in Galaxy clusters generated by structure formation

“…7 In addition, the kinetic energy flux itself has important physical meaning. It bounds the dissipation rate of kinetic energy of gas motions into thermal energy (e.g., Zhuravleva et al 2016) and into cosmic ray energy (e.g., Brunetti & Jones 2014;Miniati 2015). That energy flux also feeds the amplification of magnetic fields via small-scale dynamo action (e.g., Porter et al 2015), although on smaller scales than we simulate here (e.g., Beresnyak & Miniati 2015).…”

“…Constraining the slope of turbulent modes at small scales in the ICM is relevant to estimate of (re)acceleration of radio emitting particles (e.g. Brunetti & Jones 2014;Miniati 2015;Brunetti 2016), but we defer a more extensive exploration of this issue to future work with our ISC sample.…”

“…Subramanian et al 2006;Brunetti & Jones 2014), it can be essential to separate solenoidal from compressive turbulent motions. For example, in recent work Miniati (2015) showed that the cluster-wide ICM compressive turbulence component is likely to have a steep (Burgers-law-like) spectrum, greatly reducing the power available for cosmic ray acceleration compared to a Kraichnan-like spectrum unless that power can cascade to very small scales, where it can more efficiently transfer energy to the cosmic rays.…”

Limiting magnetic fields in the cosmic web with diffuse radio emission

“…Another indirect method is to observe the nonthermal radio synchrotron emission generated by the rela-E-mail: shea-brown@uiowa.edu tivistic plasma believed to coexist with the WHIM (Keshet et al 2004;Pfrommer et al 2006;Ryu et al 2008;Skillman et al 2008;Araya-Melo et al 2012;Vazza et al 2014Vazza et al , 2015a. If this plasma is detected at radio wavelengths, it can constrain the origin of the large-scale magnetic fields that permeate the intergalactic medium of galaxy clusters (see Brunetti & Jones 2014, for a review), differentiating, e.g., between the interaction of radio and star-forming galaxies with the WHIM (e.g. Donnert et al 2009) and a primordial magnetic field (Marinacci et al 2015).…”

How unusual is the cool-core radio halo cluster CL1821+643?

“…Theoretical models invoking reacceleration of seed electrons through a cascade of MHD turbulence introduced in the ICM by merger best explain the observed properties of radio haloes (see Donnert et al 2013;Brunetti & Jones 2014, for reviews). In addition, relativistic electrons that are secondary products of hadronic interactions in the ICM also contribute but are not sufficient (e. g. Donnert et al 2010).…”