Galaxy clusters are dynamical systems that have not reached equilibrium yet. They continue to grow by accreting matter from the cosmic web and through the mergers with smaller clusters and galaxy groups. These processes create shocks, stir bulk and turbulent motions of the gas, and make gas inhomogeneous. Many merging clusters host radio halos and relics produced by relativistic electrons re-accelerated by turbulence and shocks in the intracluster medium.
X-ray/Chandra image of MACS J0717.5+3745 galaxy cluster with radio emission contours from LOFAR. This is the most striking case of radio emission being offset from the X-ray emission. The amount of energy in turbulence estimated from the X-ray surface brightness fluctuations suggests a different ratio of kinetic over thermal energy in the regions with and without radio halo. This supports the model of turbulent acceleration of relativistic particles. Credit: A. Bonafede, M. Brueggen, D. Rafferty, I. Zhuravleva et al., MNRAS, 478, 2927, 2018.
A projected image of a galaxy cluster from cosmological numerical simulations Omega500 (K. Nelson, E. T. Lau, D. Nagai et al., ApJ, 782, 107, 2014) and the ratio of gas density fluctuations and velocity of gas motions driven by mergers throughout the cluster evolution. The averaged over a sample of similarly-relaxed clusters ratio is ~ 1 with a scatter of ~ 30%. This relation allows us to measure the velocity power spectrum indirectly using only imaging data. Future high-resolution spectroscopic data will allow us to further calibrate this relation. Credit: I. Zhuravleva et al., ApJ, 788, L13, 2014.