Diffuse Gas in Galaxy Clusters: Thermal and Non-thermal Properties

Dear Colleagues,

About 50 years ago, the presence of the diffuse gas that permeates galaxy clusters was established thanks to X-ray observations, complementing the earlier discovery of galaxies assembling into clusters themselves. Since then, tremendous observational and theoretical achievements have been accomplished, which enable us to understand the physical properties of galaxy clusters and their intracluster medium (ICM). Today, galaxy clusters are known to form at the intersection of filaments in the cosmic web and trace the peaks of the matter density field. As such, they are recognized as one of the most important cosmological probes, are used to address questions in fundamental physics, and serve as unique cosmic laboratories for astrophysics.

The observational properties of clusters reflect their hierarchical formation under gravitational collapse. At first order, they are thus fully characterized by their masses and redshift, as self-similar objects. However, clusters are also affected by complex physical processes related to gravity-induced dynamics and galaxy formation. Indeed, clusters grow via the accretion of surrounding structures and from the merging of subclusters. These energetic events dissipate the kinetic energy into heat via shocks and turbulence, but can also accelerate cosmic rays up to very high energy in the magnetized medium. Direct additional energy injection is also expected from AGN feedback, but the detailed mechanisms at play are still to be understood.

Therefore, while the overall picture depicting the intracluster medium physics has become clearer during the past decade, many new questions have also emerged about the thermal and non-thermal properties of the diffuse gas in clusters and its interplay with large scale structures and galaxy formation. This Special Issue aims to provide an update on the latest observational and theoretical developments in understanding the physical properties of the ICM. The topics include (but are not limited to):

• the observations of galaxy clusters from radio to gamma-rays
• the latest theoretical and numerical simulations developments
• the physics of the thermal plasma and its evolution with mass and redshift
• the internal structure of the matter and the thermodynamic properties of clusters
• the clusters connection to filaments and the cosmic web
• the properties of the non-thermal components, i.e., cosmic rays, magnetic field, and gas motions
• the role of AGN feedback in cluster physics and its connection to the ICM

This Special Issue is part of the ‘Galaxies and Clusters’ section. While it focuses on the properties of the diffuse gas phase in massive clusters, the other Special Issues entitled ‘The Galaxy Evolution - Evolving Cluster Connection’ and ‘The Physical Properties of the Groups of Galaxies’ provide a highly complementary view of the topic.

Dr. Rémi Adam
Guest Editor

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• galaxy clusters: astrophysics
• multiwavelength observations: from radio to gamma-rays
• numerical simulations
• Intra Cluster Medium (ICM): thermal and non-thermal physics
• feedback from active galactic nuclei
• large scale structures and the cosmic web