Objectives

Most of the baryonic matter of galaxy clusters is in the form of hot gas in the intracluster medium (ICM). The ICM has been recognized as a vital element in the formation and evolution of the large-scale structure of the universe and individual galaxies, as well as an essential diagnostic tool when studying galaxy clusters. It is highly dynamic and also contains nonthermal components such as magnetic fields and cosmic-ray protons and electrons. In this proposed Focus Meeting, among cross-communities of radio, X-ray, and SZ observations and theory, we will initiate discussions that define the roles of turbulence, shocks and cold/sloshing fronts in the ICM, as well as establish reliable, discriminating observational signatures of their properties and inter-relationships. In particular, we will address critical observational and theoretical challenges in the coming decades, specifically how emerging and planned observational efforts, such as LOFAR, uGMRT, MeerKAT, ASKAP, SKA, ngVLA (radio), eROSITA, XRISM, ATHENA (X-ray), and ACT, SPT, Planck, MUSTANG2 on the 100-meter GBT, NIKA2 on the IRAM 30-meter, ALMA + ACA, CCAT-prime, TolTEC on the 50-m LMT/GTM, Simons Observatory, APEX/CONCERTO, CMB-S4, AtLAST, LST, and potential new space-based CMB probes (SZ), can be combined with theoretical understandings to enable a comprehensive picture of the ICM. The need for such a gathering is motivated by revelations of the importance of the dynamics of the ICM through recent observations, especially in radio, X-ray, and SZ, but also in other bands, as well as developments in simulation and theory hinting at possible interpretations. The current understanding is still very incomplete. However, observations with coming facilities, together with signals from multi-messenger astronomy, and rapid progress in simulation and theory have the potential to resolve fundamental blocks to the understanding of the nature of the ICM. Now is the time to begin discussions to build a solid foundation for those developments.