Rationale

Galaxy clusters are the largest and most massive bound systems in the universe. As the nodes of large-scale structure, their formation history reflects the underlying cosmology of the universe in which they form. Although dominated by dark matter, the hot, diffuse gas in the ICM is the largest baryonic component of galaxy clusters. Although the ICM can be characterized approximately by hydrostatic equilibrium, it is far from static, but highly dynamic. Its thermodynamic properties, together with very recently available direct measurements of flow motions and magnetic fields, provide an invaluable record of the halo formation history and of the embedded galaxies. ICM motions also transport and re-distribute the cluster formation energy and the by-products and outflows from galaxies, including their active nuclei. Large-scale motions drive shocks and turbulence, evidently amplifying magnetic fields of still-uncertain origins and often accelerating (or reaccelerating) relativistic, cosmic ray electrons that “light up” the ICM with diffuse radio synchrotron emission as they interact with the ICM magnetic field. Cosmic ray protons are also presumably injected and accelerated. Their presence is so far unconfirmed, but current upper limits still allow them to play potentially important energetic roles within the ICM.

The properties and distributions of these ICM components provide unique probes of cluster evolution and its physics. The relationships between the “thermal gas” and the “non-thermal components” provide powerful probes of the physics of the ICM, which are quite distinct in important properties from those accessible in the laboratory, the heliosphere or the interstellar medium. Existing ICM observations, especially, but not exclusively, in the radio and X-ray bands, have revealed the presence of these components and hinted at tell-tale patterns connecting to cluster physics. Similarly, existing computer simulations and analytic theory demonstrated that the non-thermal components have the potential to provide essential diagnostics of cluster and ICM dynamics that are distinct from those available from observations only of the dominant “thermal” component. 

At the moment, however, we have far more questions than answers. But the new generations of developing and planned radio, X-ray, and SZ observatories will be capable of enabling fundamental steps forward. Some newly functioning radio observatories, such as LOFAR, MWA, MeerKAT and ASKAP, are already beginning to reveal this potential. Other radio facilities under development or in planning, such as SKA and ngVLA, can go much further in revealing ICM cosmic ray electron populations and magnetic field distributions, cluster radio AGN behaviors, and the various inter-relationships among these constituents. The new generations of X-ray telescopes, such as eROSITA, XRISM, and especially ATHENA, will be able to characterize the ICM dynamics much more completely than is possible at present. Similarly, improvements in the measurements and interpretation of kinetic SZ afforded, for example, by ACT, SPT, MUSTANG2 on the 100-meter GBT, NIKA2 on the IRAM 30-meter, ALMA + ACA, and TolTEC on the 50-meter LMT, are opening up new paths to characterizing ICM bulk motions, while proposed and planned observatories such as AtLAST, LST, CCAT-prime, the Simons Observatory, CMB-HD, and CMB-S4 will revolutionize SZ studies. In the next decade, signals from multi-wavelength, multi-messenger astronomy, i.e., gamma rays, ultra-high energy cosmic rays and neutrinos (potentially detectable, for example, by CTA, HAWC, Auger, TA, IceCube, ANITA, or successors), will provide a complementary and necessary insight into the physics of ICM and on the nature of dark matter. Simultaneously, rapid improvements in computing power and algorithms as well as advances in theory are enabling much improved modeling of ICM dynamics and associated physics.

Now is the time to initiate cross-community discussions of these issues, enabling essential coordination and cross-community interactions, so that we will be prepared in the coming decades to take the full advantage of these multiple advances in observations and theory.