# On the absence of radio halos in clusters with double relics

## TL;DR

This study investigates why some galaxy clusters with double relics lack radio halos, using new upper limit methods and multi-wavelength data to explore the connection between merger dynamics and radio emission.

## Contribution

The paper introduces a novel method to set upper limits on radio halo emission and analyzes various data to understand the conditions suppressing radio halos in clusters with double relics.

## Key findings

- Clusters with double relics are more disturbed and differ in radio properties.
- No significant impact of merger mass ratio on radio halo presence.
- Absence of radio halos may relate to merger stage, but data is limited.

## Abstract

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