Evolutionary phases of merging clusters as seen by LOFAR

Abstract

Massive, merging galaxy clusters often host giant, diffuse radio sources that arise from shocks and turbulence; hence, radio observations can be useful for determining the merger state of a cluster. In preparation for a larger study, we selected three clusters - Abell 1319, Abell 1314, and RXC J1501.3+4220 (Z7215) - making use of the new LOFAR Two-Metre Sky Survey (LoTSS) at 120-168 MHz, and together with archival data, show that these clusters appear to be in pre-merging, merging, and post-merging states, respectively. We argue that Abell 1319 is likely in its pre-merging phase, where three separate cluster components are about to merge. There are no radio haloes nor radio relics detected in this system. Abell 1314 is a highly disturbed, low-mass cluster that is likely in the process of merging. This low-mass system does not show a radio halo, however, we argue that the merger activates mechanisms that cause electron re-acceleration in the large 800 kpc radio tail associated with IC 711. In the cluster Z7215 we discover diffuse radio emission at the cluster centre, and we classify this emission as a radio halo, although it is dimmer and smaller than expected by the radio halo power versus cluster mass correlation. We suggest that the disturbed cluster Z7215 is in its post-merging phase. Systematic studies of this kind over a larger sample of clusters observed with LoTSS will help to constrain the timescales involved in turbulent re-acceleration and the subsequent energy losses of the underlying electrons.