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