How merger-driven gas motions in galaxy clusters can turn AGN bubbles into radio relics

TL;DR

This paper demonstrates through simulations that merger-driven gas motions in galaxy clusters can transport and shape relativistic electrons into filamentary regions, which, upon shock passage, produce radio relics with high polarization, independent of shock location.

Contribution

It introduces a novel explanation for radio relics as shaped by gas motions and seed electron distributions, rather than solely by shock fronts.

Findings

Gas motions advect cosmic rays to large radii.

Relativistic electrons are spread tangentially, forming filamentary regions.

Radio relic edges may trace electron distribution, not shock fronts.

Abstract

Radio relics in galaxy clusters are extended synchrotron sources produced by cosmic-ray electrons in the $\mu$G magnetic field. Many relics are found in the cluster periphery and have a cluster-centric, narrow arc-like shape, which suggests that the electrons are accelerated or re-accelerated by merger shock fronts propagating outward in the intracluster plasma. In the X-ray, some relics do exhibit such shocks at the location of the relic, but many do not. We explore the possibility that radio relics trace not the shock fronts but the shape of the underlying distribution of seed relativistic electrons, lit up by a recent shock passage. We use magnetohydrodynamic simulations of cluster mergers and include bubbles of relativistic electrons injected by jets from the central AGN or from an off-center radio galaxy. We show that the merger-driven gas motions (a) can advect the bubble cosmic rays to very large radii, and (b) spread the relativistic seed electrons preferentially in tangential direction -- along the gravitational equipotential surfaces -- producing extended, filamentary or sheet-like regions of intracluster plasma enriched with aged cosmic rays, which resemble radio relics. Once a shock front passes across such a region, the sharp radio emission edges would trace the sharp boundaries of these enriched regions rather than the front. We also show that these elongated cosmic ray features are naturally associated with magnetic fields stretched tangentially along their long axis, which could help explain the high polarization of relics.