Occurrence of radio halos in galaxy clusters

Insight from a mass-selected sample

¹ Dipartimento di Fisica e Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
² INAF-Istituto di Radioastronomia, via P. Gobetti 101, 40129 Bologna, Italy
e-mail: vcuciti@ira.inaf.it
³ National Centre of Radio Astrophysics, TIFR, 411007 Pune, India
⁴ INAF/Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁵ INFN, Sezione di Bologna, viale Berti Pichat 6/2, 40127, Bologna, Italy

Received: 27 April 2015
Accepted: 8 June 2015

Abstract

Context. Giant radio halos (RH) are diffuse Mpc-scale synchrotron sources detected in some massive and merging galaxy clusters. An unbiased study of the statistical properties of RHs is crucial to constraining their origin and evolution.

Aims. We investigate the occurrence of RHs and its dependence on the cluster mass in a Sunyaev-Zel’dovich (SZ)-selected sample of galaxy clusters, which is nearly a mass-selected sample. Moreover, we analyse the connection between RHs and merging clusters.

Methods. We selected from the Planck SZ catalogue clusters with M ≥ 6 × 10¹⁴M_⊙ at z = 0.08−0.33 and we searched for the presence of RHs using the NVSS for z < 0.2 and the GMRT RH survey (GRHS) and its extension (EGRHS) for 0.2 < z < 0.33. We used archival Chandra X-ray data to derive information on the dynamical status of the clusters.

Results. We confirm that RH clusters are merging systems while the majority of clusters without RH are relaxed, thus supporting the idea that mergers play a fundamental role in the generation of RHs. We find evidence for an increase in the fraction of clusters with RHs with the cluster mass and this is in line with expectations derived on the basis of the turbulence re-acceleration scenario. Finally, we discuss the effect of the incompleteness of our sample on this result.