On the absence of radio haloes in clusters with double relics

Abstract

Pairs of radio relics are believed to form during cluster mergers, and are\nbest observed when the merger occurs in the plane of the sky. Mergers can also\nproduce radio halos, through complex processes likely linked to turbulent\nre-acceleration of cosmic-ray electrons. However, only some clusters with\ndouble relics also show a radio halo. Here, we present a novel method to derive\nupper limits on the radio halo emission, and analyse archival X-ray Chandra\ndata, as well as galaxy velocity dispersions and lensing data, in order to\nunderstand the key parameter that switches on radio halo emission. We place\nupper limits on the halo power below the $P_{\\rm 1.4 \\, GHz}\\, M_{500}$\ncorrelation for some clusters, confirming that clusters with double relics have\ndifferent radio properties. Computing X-ray morphological indicators, we find\nthat clusters with double relics are associated with the most disturbed\nclusters. We also investigate the role of different mass-ratios and\ntime-since-merger. Data do not indicate that the merger mass ratio has an\nimpact on the presence or absence of radio halos (the null hypothesis that the\nclusters belong to the same group cannot be rejected). However, the data\nsuggests that the absence of radio halos could be associated with early and\nlate mergers, but the sample is too small to perform a statistical test. Our\nstudy is limited by the small number of clusters with double relics. Future\nsurveys with LOFAR, ASKAP, MeerKat and SKA will provide larger samples to\nbetter address this issue.\n