A formation mechanism for "Wrong Way" Radio Relics

TL;DR

This paper proposes a new formation mechanism for 'Wrong Way' Radio Relics, where inward-bent shock waves caused by infalling galaxy clusters produce atypical relic morphologies and spectral properties, supported by high-resolution simulations.

Contribution

It introduces a novel formation scenario for 'Wrong Way' Radio Relics involving inward-bent shocks, validated through detailed spectral simulations of galaxy cluster mergers.

Findings

Inward-bent shocks can produce 'Wrong Way' relic morphologies.

Spectral index variations are explained by shock bending effects.

Simulation results match observed atypical relic features.

Abstract

Radio Relics are typically found to be arc-like regions of synchrotron emission in the outskirts of merging galaxy clusters, bowing out from the cluster center. In most cases they show synchrotron spectra that steepen towards the cluster center, indicating that they are caused by relativistic electrons being accelerated at outwards traveling merger shocks. A number of radio relics break with this ideal picture and show morphologies that are bent the opposite way and show spectral index distributions which do not follow expectations from the ideal picture. We propose that these `Wrong Way' Relics can form when an outwards travelling shock wave is bent inwards by an in-falling galaxy cluster or group. We test this in an ultra-high resolution zoom-in simulation of a massive galaxy cluster with an on-the-fly spectral Cosmic Ray model. This allows us to study not only the synchrotron emission at colliding shocks, but also their synchrotron spectra to adress the open question of relics with strongly varying spectral indices over the relic surface.