Diffuse radio emission in MACS J1752.0+4440

TL;DR

This paper reports the discovery of large-scale diffuse radio emission, including double relics and a radio halo, in galaxy cluster MACS J1752.0+4440, indicating a recent binary merger with shock acceleration of particles.

Contribution

First detailed radio observation of MACS J1752.0+4440 revealing double relics and a halo, supporting models of shock acceleration in cluster mergers.

Findings

Double relics and a radio halo detected in MACS J1752.0+4440.

Relic sizes suggest a 2:1 mass ratio in the merger.

Spectral indices indicate shock Mach numbers of 3.5 to 4.5.

Abstract

We report the discovery of large-scale diffuse radio emission in the galaxy cluster MACS J1752.0+4440 (z=0.366). Using Westerbork Synthesis Radio Telescope (WSRT) observations we find that the cluster hosts a double radio relic system as well as a 1.65 Mpc radio halo covering the region between the two relics. The relics are diametrically located on opposite sides of the cluster center. The NE and SW relics have sizes of 1.3 and 0.9 Mpc, respectively. In case of an isolated binary merger event, the relative sizes of the relics suggest a mass ratio about 2:1. We measure integrated spectra of -1.16 \pm 0.03 for the NE and -1.10 \pm 0.05 for the SW relic. We conclude that this cluster has undergone a violent binary merger event and the relics are best explained by particles (re)accelerated in outwards traveling shock waves. The spectral indices suggest the relics trace shock waves with Mach numbers (M) of around 3.5 to 4.5. These relatively high Mach numbers derived from the radio spectral index are comparable to those derived for a few other recently discovered relics. This implies that merger shocks with M > 3 are relatively common in cluster outskirts if our understanding of diffusive shock acceleration is correct.