Radio halos in a mass-selected sample of 75 galaxy clusters

II. Statistical analysis

¹ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
e-mail: vcuciti@hs.uni-hamburg.de
² INAF-Istituto di Radioastronomia, via P. Gobetti 101, 40129 Bologna, Italy
³ Dipartimento di Fisica e Astronomia, Università di Bologna, via P. Gobetti 93/2, 40129 Bologna, Italy
⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁵ National Centre for Radio Astrophysics, Tata Institute of Fundamental Research Savitribai Phule Pune University Campus, Pune, 411 007 Maharashtra, India
⁶ AIM, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, 91191 Gif-sur-Yvette, France
⁷ INAF, Osservatorio di Astrofisica e Scienza dello Spazio, via Pietro Gobetti 93/3, 40129 Bologna, Italy
⁸ INFN, Sezione di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy
⁹ Naval Research Laboratory, 4555 Overlook Avenue SW, Code 7213, Washington, DC 20375, USA

Received: 18 August 2020
Accepted: 5 January 2021

Abstract

Context. Many galaxy clusters host megaparsec-scale diffuse radio sources called radio halos. Their origin is tightly connected to the processes that lead to the formation of clusters themselves. In order to reveal this connection, statistical studies of the radio properties of clusters combined with their thermal properties are necessary. For this purpose, we selected a sample of galaxy clusters with M₅₀₀ ≥ 6 × 10¹⁴ M_⊙ and z = 0.08 − 0.33 from the Planck Sunyaev–Zel’dovich catalogue. In Paper I, we presented the radio and X-ray data analysis that we carried out on the clusters of this sample.

Aims. In this paper we exploit the wealth of data presented in Paper I to study the radio properties of the sample, in connection to the mass and dynamical state of clusters.

Methods. We used the dynamical information derived from the X-ray data to assess the role of mergers in the origin of radio halos. We studied the distribution of clusters in the radio power–mass diagram, the scaling between the radio luminosity of radio halos and the mass of the host clusters, and the role of dynamics in the radio luminosity and emissivity of radio halos. We measured the occurrence of radio halos as a function of the cluster mass and we compared it with the expectations of models developed in the framework of turbulent acceleration.

Results. We find that more than the 90% of radio halos are in merging clusters and that their radio power correlates with the mass of the host clusters. The correlation shows a large dispersion. Interestingly, we show that cluster dynamics contributes significantly to this dispersion, with more disturbed clusters being more radio luminous. Clusters without radio halos are generally relaxed, and the upper limits to their diffuse emission lie below the correlation. Moreover, we show that the radio emissivity of clusters exhibits an apparent bimodality, with the emissivity of radio halos being at least ∼5 times larger than the non-emission associated with more relaxed clusters. We find that the fraction of radio halos drops from ∼70% in high-mass clusters to ∼35% in the lower mass systems in the sample and we show that this result is in good agreement with the expectations from turbulent re-acceleration models.