Abstract
In a growing number of galaxy clusters diffuse extended radio sources have been found. These sources are not directly associated with individual cluster galaxies. The radio emission reveal the presence of cosmic rays and magnetic fields in the intracluster medium (ICM). We classify diffuse cluster radio sources into radio halos, cluster radio shocks (relics), and revived AGN fossil plasma sources. Radio halo sources can be further divided into giant halos, mini-halos, and possible “intermediate” sources. Halos are generally positioned at cluster center and their brightness approximately follows the distribution of the thermal ICM. Cluster radio shocks (relics) are polarized sources mostly found in the cluster’s periphery. They trace merger induced shock waves. Revived fossil plasma sources are characterized by their radio steep-spectra and often irregular morphologies. In this review we give an overview of the properties of diffuse cluster radio sources, with an emphasis on recent observational results. We discuss the resulting implications for the underlying physical acceleration processes that operate in the ICM, the role of relativistic fossil plasma, and the properties of ICM shocks and magnetic fields. We also compile an updated list of diffuse cluster radio sources which will be available on-line (http://galaxyclusters.com). We end this review with a discussion on the detection of diffuse radio emission from the cosmic web.
Highlights
Galaxy clusters are the largest virialized objects in our Universe, with masses up to ∼ 1015 M
Galaxy clusters provide a unique environment to study the physics of particle acceleration in collisionless, high-β, turbulent plasmas, where β is the ratio of the thermal pressure to the magnetic pressure,1 and at low Mach numbers shocks
Revived active galactic nuclei (AGN) fossil plasma sources, phoenices, and Gently re-energized tails (GReETs) In this class we group sources that trace AGN radio plasma that has somehow been re-energized through processes in the intracluster medium (ICM), unrelated to the radio galaxy itself
Summary
Galaxy clusters are the largest virialized objects in our Universe, with masses up to ∼ 1015 M. Most of the baryonic mass of clusters is contained in a hot (107–108 K) ionized intracluster medium (ICM), held together by the clusters’ gravitational pull This dilute magnetized plasma (∼ 10−3 particles cm−3) emits thermal Bremsstrahlung at X-ray wavelengths, permeating the cluster’s volume (e.g., Mitchell et al 1976; Serlemitsos et al 1977; Forman and Jones 1982), see Fig. 1. The radio plasma has displaced the X-ray emitting gas, creating a low-density bubble which rises buoyantly and expands, distributing energy to the surrounding ICM (e.g., Churazov et al 2002). This process is commonly referred to as “radio-mode” feedback, it is still being debated what the precise mechanism is that transfers the energy to the ICM
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