Galaxy clusters host a large reservoir of very hot, low-density and weakly-magnetized plasma that fills the space between galaxies. Thanks to the large sizes of clusters and their relative simplicity, it is possible to probe microphysical properties of this plasma, such as turbulence, heat conductivity, and gas viscosity. Related questions are the acceleration of particles and the energy exchange between the particle populations and the magnetic field. While these are microphysical processes, they affect large-scale phenomena, including feedback from supermassive black holes, mergers with smaller clusters and groups, thermodynamic evolution of galaxies and galaxy clusters.
X-ray image of the Coma cluster observed with Chandra (top). Dashed lines show the regions used to probe the behavior of particles on scales comparable to their mean free path. On these scales, the effects of magnetic fields and plasma instabilities should manifest themselves. The amplitude of density fluctuations (red and blue regions, bottom panel) measured in the dashed regions are compared with the results of numerical simulations (dashed and solid curves). One expects that the small-scale (large wavenumber) fluctuations will be suppressed if the gas viscosity and conduction as in unmagnetized plasma. Such suppression is clearly inconsistent with the observed data, meaning that the isotropic transport processes are suppressed in the plasma likely due to the presence of weak magnetic fields and plasma instabilities. Credit: Zhuravleva et al. 2019, Nature Astronomy, 2019, 3, 382.
X-ray/XMM-Newton image of the core of the Virgo cluster. The white square indicates a sharp surface brightness discontinuity associated with a sloshing cold front. The sharpest part of the front has a width that is smaller than the mean free path of particles across the cold front. This indicates that diffusion, conduction and mixing processes are suppressed. This happens naturally when magnetic fields are aligned with the front surface. The zoomed-in panel shows the map of surface brightness fluctuations that reveals three bright quasi-linear features. Comparison with numerical simulations suggests that these are local amplifications of magnetic fields by sloshing in wide layers below the cold front. Credit: N. Werner, J. ZuHone, I. Zhuravleva et al., MNRAS, 455, 846, 2016.