Thomas W. Jones

Cosmic rays can be accelerated in galaxy clusters by different mechanisms and remain confined in the cluster volume accumulating for cosmological times. This component is expected to generate non-thermal radiation from radio to γ-rays through a variety of mechanisms. Mpc-scale synchrotron radiation from the inter-galactic-medium is nowadays observed in many nearby, massive, clusters and provides a probe of the complex interplay between thermal gas, magnetic fields and cosmic rays in galaxy clusters. The interaction of cosmic rays with magnetic fields is of primary importance for the acceleration, evolution and dynamics of these particles. Cosmic rays are trapped and accelerated via the scattering with magnetic field fluctuations in converging (shocks) or turbulent flows driven, at least in part, by the hierarchical process of clusters formation. Interestingly, this also connects the processes of cluster formation and particle acceleration in the intra-cluster-medium. In this chapter we describe the basic ingredients of the physics of cosmic rays in galaxy clusters and report on the most relevant observables that are nowadays used for constraining their origin and evolution.

Recent radio observations have identified a class of structures, so-called radio relics, in clusters of galaxies. The radio emission from these sources is interpreted as synchrotron radiation from GeV electrons gyrating in μG-level magnetic fields. Radio relics, located mostly in the outskirts of clusters, seem to associate with shock waves, especially those developed during mergers. In fact, they seem to be good structures to identify and probe such shocks in intracluster media (ICMs), provided we understand the electron acceleration and re-acceleration at those shocks. In this paper, we describe time-dependent simulations for diffusive shock acceleration at weak shocks that are expected to be found in ICMs. Freshly injected as well as pre-existing populations of cosmic-ray (CR) electrons are considered, and energy losses via synchrotron and inverse Compton are included. We then compare the synchrotron flux and spectral distributions estimated from the simulations with those in two well-observed radio relics in CIZA J2242.8+5301 and ZwCl0008.8+5215. Considering that CR electron injection is expected to be rather inefficient at weak shocks with Mach number M ≲ a few, the existence of radio relics could indicate the pre-existing population of low-energy CR electrons in ICMs. The implication of our results on the merger shock scenario of radio relics is discussed.

We present a pair of three-dimensional magnetohydrodynamical simulations of intermittent jets from a central active galactic nucleus (AGN) in a galaxy cluster extracted from a high-resolution cosmological simulation. The selected cluster was chosen as an apparently relatively relaxed system, not having undergone a major merger in almost 7 Gyr. Despite this characterization and history, the intracluster medium (ICM) contains quite active “weather.” We explore the effects of this ICM weather on the morphological evolution of the AGN jets and lobes. The orientation of the jets is different in the two simulations so that they probe different aspects of the ICM structure and dynamics. We find that even for this cluster, which can be characterized as relaxed by an observational standard, the large-scale, bulk ICM motions can significantly distort the jets and lobes. Synthetic X-ray observations of the simulations show that the jets produce complex cavity systems, while synthetic radio observations reveal bending of the jets and lobes similar to wide-angle tail radio sources. The jets are cycled on and off with a 26 Myr period using a 50% duty cycle. This leads to morphological features similar to those in “double–double” radio galaxies. While the jet and ICM magnetic fields are generally too weak in the simulations to play a major role in the dynamics, Maxwell stresses can still become locally significant.

We present a consistent three-dimensional model for the head-tail radio galaxy NGC 1265 that explains the complex radio morphology and spectrum by a past passage of the galaxy and radio bubble through a shock wave. Using analytical solutions to the full Riemann problem and hydrodynamical simulations, we study how this passage transformed the plasma bubble into a toroidal vortex ring. Adiabatic compression of the aged electron population causes it to be energized and to emit low surface brightness and steep-spectrum radio emission. The large infall velocity of NGC 1265—which is barely gravitationally bound to the Perseus cluster at its current position—and the low Faraday rotation measure values and variance of the jet strongly argue that this transformation was due to the accretion shock onto Perseus situated roughly at R₂₀₀. Estimating the volume change of the radio bubble enables inferring a shock Mach number of ℳ ≃ 4.2^+0.8_–1.2, a density jump of 3.4^+0.2_–0.4, a temperature jump of 6.3^+2.5_–2.7, and a pressure jump of 21.5 ± 10.5 while allowing for uncertainties in the equation of state of the radio plasma and volume of the torus. Extrapolating X-ray profiles, we obtain upper limits on the gas temperature and density in the infalling warm-hot intergalactic medium of kT ≲ 0.4 keV and n ≲ 5 × 10^–5 cm^–3. The orientation of the ellipsoidally shaped radio torus in combination with the direction of the galaxy’s head and tail in the plane of the sky is impossible to reconcile with projection effects. Instead, this argues for post-shock shear flows that have been caused by curvature in the shock surface with a characteristic radius of 850 kpc. The energy density of the shear flow corresponds to a turbulent-to-thermal energy density of 14%—consistent with cosmological simulations. The shock-injected vorticity might be important in generating and amplifying magnetic fields in galaxy clusters. We suggest that future polarized radio observations by, e.g., LOFAR of head-tail galaxies can be complementary probes of accretion shocks onto galaxy clusters and are unique in determining their detailed flow properties.

We present calculations of expected continuum emissions from Sedov–Taylor phase Type Ia supernova remnants (SNRs), using the energy spectra of cosmic ray (CR) electrons and protons from non-linear diffusive shock acceleration simulations. A new, general-purpose radiative process code, cosmicp, was employed to calculate the radiation expected from CR electrons and protons and their secondary products. These radio, X-ray and gamma-ray emissions are generally consistent with current observations of Type Ia SNRs. The emissions from electrons in these models dominate the radio through X-ray bands. Decays of π⁰s from p–p collisions mostly dominate the gamma-ray range, although for a hot, low-density ISM case (n_ISM = 0.003 cm⁻³), the pion decay contribution is reduced sufficiently to reveal the inverse Compton contribution to TeV gamma-rays. In addition, we present simple scalings for the contributing emission processes to allow a crude exploration of model parameter space, enabling these results to be used more broadly. We also discuss the radial surface brightness profiles expected for these model SNRs in the X-ray and gamma-ray bands.

We report on a series of three-dimensional magnetohydrodynamic simulations of active galactic nucleus (AGN) jet propagation in realistic models of magnetized galaxy clusters. We are primarily interested in the details of energy transfer between jets and the intracluster medium (ICM) to help clarify what role such flows could have in the reheating of cluster cores. Our simulated jets feature a range of intermittency behaviors, including intermittent jets that periodically switch on and off and one model jet that shuts down completely, naturally creating a relic plume. The ICM into which these jets propagate incorporates tangled magnetic field geometries and density substructure designed to mimic some likely features of real galaxy clusters. We find that our jets are characteristically at least 60% efficient at transferring thermal energy to the ICM. Irreversible heat energy is not uniformly distributed, however, instead residing preferentially in regions very near the jet/cocoon boundaries. While intermittency affects the details of how, when, and where this energy is deposited, all of our models generically fail to heat the cluster cores uniformly. Both the detailed density structure and nominally weak magnetic fields in the ICM play interesting roles in perturbing the flows, particularly when the jets are non-steady. Still, this perturbation is never sufficient to isotropize the jet energy deposition, suggesting that some other ingredient is required for AGN jets to successfully reheat cluster cores.

We propose a model for diffusive shock acceleration (DSA) in which stochastic magnetic fields in the shock precursor are generated through purely fluid mechanisms of a so-called small-scale dynamo. This contrasts with previous DSA models that considered magnetic fields amplified through cosmic ray (CR) streaming instabilities, i.e., either by way of individual particles resonant scattering in the magnetic fields, or by macroscopic electric currents associated with large-scale CR streaming. Instead, in our picture, the solenoidal velocity perturbations that are required for the dynamo to work are produced through the interactions of the pressure gradient of the CR precursor and density perturbations in the inflowing fluid. Our estimates show that this mechanism provides fast growth of magnetic field and is very generic. We argue that for supernovae shocks the mechanism is capable of generating upstream magnetic fields that are sufficiently strong for accelerating CRs up to around 10¹⁶ eV. No action of any other mechanism is necessary.