Letters of Intent received in 2016

LoI 2018-1901
Focus Meeting (GA): Nonthermal Phenomena in Galaxy Clusters: New Frontiers

Topics

1) ICM Shocks and Turbulence
2) ICM Magnetic Fields
3) Galaxy cluster Cosmic Rays
4) Galaxy cluster AGNs
5) Non-equilibrium plasma processes in galaxy clusters
6) Relationship between ICM nonthermal phenomena and the galaxy cluster formation process
7) Observational properties of nonthermal phenomena in galaxy clusters

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, diffuse, very weakly collisional intracluster plasmas (ICMs) are the dominant baryonic components of clusters. Although ICMs can be characterized approximately by hydrostatic equilibrium, they are not really static. Their thermodynamic properties and chemistry, together with very recently available direct measurements of the plasma velocity distributions, provide an invaluable record of the halo formation history and of the embedded galaxies. ICM motions also transport and re-distribute cluster formation energy and the by-products and outflow from galaxies, including their active nuclei. Large-scale ICM 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 ICMs 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 ICMs.

The properties and distributions of these intracluster plasmas provide unique probes of cluster evolution and its physics. The relationships between the “thermal” and non-thermal” plasmas provide powerful probes of the physics of ICM plasmas, which are quite distinct in important properties from plasmas 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. However, new generations of developing and planned radio and X-ray observatories will be capable of enabling fundamental steps forward. Some newly functioning radio observatories, such as LOFAR, MWA, and JVLA are already beginning to reveal this potential. Other radio facilities under development or in planning, such as uGMRT, MeerKAT and ASKAP, up to 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. Coming generations of X-ray telescopes, such as eROSITA, HXMT, and especially ATHENA, will be able to characterize ICM dynamics much more completely than is possible at present. Similarly, improvements in measurements and interpretation of the kinetic Sunyaev-Zeldovich Effect afforded, for example, by statistical detections by ACT, SPT, and Planck and individual detections by MUSTANG and NIKA are opening up new paths to characterizing ICM bulk motions. Simultaneously, rapid advances in computing power and improvements in algorithms are enabling much improved modeling of ICM dynamics as well as associated plasma physics.

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