Search for low-frequency diffuse radio emission around a shock in the massive galaxy cluster MACS J0744.9+3927

TL;DR

This study used LOFAR radio observations to search for diffuse radio emission around a shock in galaxy cluster MACS J0744.9+3927, finding no evidence of relics or halos and attributing observed emissions to AGN activity.

Contribution

First deep low-frequency radio imaging of MACS J0744.9+3927, providing insights into shock-related radio emission and its origin in a high-redshift merging galaxy cluster.

Findings

No detection of cluster-scale diffuse radio emission.

Radio emission near the shock is likely from AGN activity.

The shock is too weak to produce detectable relics via diffusive shock acceleration.

Abstract

Merging galaxy clusters produce low Mach number shocks in the intracluster medium. These shocks can accelerate electrons to relativistic energies that are detectable at radio frequencies. MACS J0744.9+3927 is a massive ($M_{500} = (11.8 \pm 2.8) \times 10^{14} M_{\odot}$), high-redshift ($z=0.6976$) cluster where a Bullet-type merger is presumed to have taken place. Sunyaev-Zel'dovich maps from MUSTANG indicate that a shock, with Mach number $\mathcal{M} = 1.0-2.9$ and an extension of $\sim 200$ kpc, sits near the centre of the cluster. The shock is also detected as a brightness and temperature discontinuity in X-ray observations. To search for diffuse radio emission associated with the merger, we have imaged the cluster with the LOw Frequency ARray (LOFAR) at 120-165 MHz. Our LOFAR radio images reveal previously undetected AGN emission, but do not show clear cluster-scale diffuse emission in the form of a radio relic nor a radio halo. The region of the shock is on the western edge of AGN lobe emission from the brightest cluster galaxy. Correlating the flux of known shock-induced radio relics versus their size, we find that the radio emission overlapping the shocked region in MACS J0744.9+3927 is likely of AGN origin. We argue against the presence of a relic caused by diffusive shock acceleration and suggest that the shock is too weak to accelerate electrons from the intracluster medium.