High $\beta$ Effects on Cosmic Ray Streaming in Galaxy Clusters

Abstract

Diffuse, extended radio emission in galaxy clusters, commonly referred to as radio halos, indicate the presence of high energy cosmic ray (CR) electrons and cluster-wide magnetic fields. We can predict from theory the expected surface brightness of a radio halo, given magnetic field and CR density profiles. Previous studies have shown that the nature of CR transport can radically effect the expected radio halo emission from clusters (Wiener et al. 2013). Reasonable levels of magnetohydrodynamic (MHD) wave damping can lead to significant CR streaming speeds. But a careful treatment of MHD waves in a high $\beta$ plasma, as expected in cluster environments, reveals damping rates may be enhanced by a factor of ${\beta }^{1/2}$. This leads to faster CR streaming and lower surface brightnesses than without this effect. In this work we re-examine the simplified, 1D Coma cluster simulations (with radial magnetic fields) of Wiener et al. (2013) and discuss observable consequences of this high $\beta$ damping. Future work is required to study this effect in more realistic simulations.

Accepted by MNRAS

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• Shocks and non-thermal particles in clusters of galaxies
• The impact of astrophysical dust grains on the confinement of cosmic rays
• Towards First-principle Characterization of Cosmic-ray Transport Coefficients from Multi-scale Kinetic Simulations
• Stress-Testing Cosmic Ray Physics: The Impact of Cosmic Rays on the Surviving Disk of Ram Pressure Stripped Galaxies
• On the Challenges of Cosmic-Ray Proton Shock Acceleration in the Intracluster Medium