Cosmic ray confinement in fossil cluster bubbles

TL;DR

This study uses simulations to show that magnetic fields in galaxy cluster bubbles can effectively confine cosmic rays, explaining radio ghosts and influencing cluster dynamics and observable phenomena.

Contribution

It demonstrates that magnetic draping can significantly suppress cosmic ray diffusion in AGN-inflated bubbles, providing a new explanation for radio ghost longevity.

Findings

Magnetic fields can confine cosmic rays within bubbles for long timescales.

Isotropic diffusion would lead to rapid cosmic ray escape, inconsistent with observations.

Partial cosmic ray escape occurs mainly in the wake of rising bubbles, affecting cluster features.

Abstract

Most cool core clusters of galaxies possess active galactic nuclei (AGN) in their centers. These AGN inflate buoyant bubbles containing non-thermal radio emitting particles. If such bubbles efficiently confine cosmic rays (CR) then this could explain ``radio ghosts'' seen far from cluster centers. We simulate the diffusion of cosmic rays from buoyant bubbles inflated by AGN. Our simulations include the effects of the anisotropic particle diffusion introduced by magnetic fields. Our models are consistent with the X-ray morphology of AGN bubbles, with disruption being suppressed by the magnetic draping effect. We conclude that for such magnetic field topologies, a substantial fraction of cosmic rays can be confined inside the bubbles on buoyant rise timescales even when the parallel diffusivity coefficient is very large. For isotropic diffusion at a comparable level, cosmic rays would leak out of the bubbles too rapidly to be consistent with radio observations. Thus, the long confinement times associated with the magnetic suppression of CR diffusion can explain the presence of radio ghosts. We show that the partial escape of cosmic rays is mostly confined to the wake of the rising bubbles, and speculate that this effect could: (1) account for the excitation of the H$\alpha$ filaments trailing behind the bubbles in the Perseus cluster, (2) inject entropy into the metal enriched material being lifted by the bubbles and, thus, help to displace it permanently from the cluster center and (3) produce observable $\gamma$-rays via the interaction of the diffusing cosmic rays with the thermal intracluster medium (ICM).