Why are central radio relics so rare?

TL;DR

This study investigates why radio relics associated with shock acceleration are predominantly found in the outskirts of galaxy clusters, using cosmological simulations to analyze shock energy dissipation and relic detectability.

Contribution

The paper demonstrates that the radial distribution of observed relics aligns with the energy dissipation pattern of shocks, explaining their scarcity in cluster centers.

Findings

Radio relics are rarely detected within 200 kpc of cluster centers.

Most detectable relics are found beyond 800 kpc from the center.

Central relics are rare due to low surface brightness and geometrical effects.

Abstract

In this paper we address the question why cluster radio relics that are connected to shock acceleration, so-called radio gischt, have preferentially been found in the outskirts of galaxy clusters. By identifying merger shock waves in cosmological grid simulations, we explore several prescriptions for relating the energy dissipated in shocks to the energy emitted in the radio band. None of the investigated models produce detectable radio relics within 100-200 kpc from the cluster centre. All models cause > 50 per cent of the detectable relic emission at projected distances > 800 kpc. Central radio relics caused by shocks that propagate along the line-of-sight are rare events for simple geometrical reasons, and they have a low surface brightness making them elusive for current instruments. Our simulations show that the radial distribution of observed relics can be explained by the radial trend of dissipated kinetic energy in shocks, that increases with distance from the cluster centre up until half of the virial radius.