Efficient micromirror confinement of sub-teraelectronvolt cosmic rays in galaxy clusters

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Patrick Reichherzer, Archie F. A. Bott, Robert J. Ewart, Gianluca Gregori, Philipp Kempski, Matthew W. Kunz, Alexander A. Schekochihin

Published: 03 Jan 2025, Last Modified: 24 Jan 2026Nature AstronomyEveryoneRevisionsCC BY-SA 4.0
Abstract: Cosmic rays (CRs) play a pivotal role in shaping the thermal and dynamical properties of astrophysical environments, such as galaxies and galaxy clusters. Recent observations suggest a stronger confinement of CRs in certain astrophysical systems than predicted by current CR-transport theories. Here, we show that the incorporation of microscale physics into CR-transport models can account for this enhanced CR confinement. We develop a theoretical description of the effect of magnetic microscale fluctuations originating from the mirror instability on macroscopic CR diffusion. We confirm our theory with large-dynamical-range simulations of CR transport in the intracluster medium (ICM) of galaxy clusters and kinetic simulations of CR transport in micromirror fields. We conclude that sub-teraelectronvolt CR confinement in the ICM is far more effective than previously anticipated on the basis of Galactic-transport extrapolations. The transformative impact of micromirrors on CR diffusion provides insights into how microphysics can reciprocally affect macroscopic dynamics and observable structures across a range of astrophysical scales. Microscale fluctuations originating from the mirror instability can effectively keep sub-teraelectronvolt cosmic rays confined to galaxy clusters and explain recent observations.