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

Abstract

Context. Radio halos are synchrotron diffuse sources at the centre of a fraction of galaxy clusters. The study of large samples of clusters with adequate radio and X-ray data is necessary to investigate the origin of radio halos and their connection with the cluster dynamics and formation history. Aims. The aim of this paper is to compile a well-selected sample of galaxy clusters with deep radio observations to perform an unbiased statistical study of the properties of radio halos. Methods. We selected 75 clusters with M ≥ 6 × 1014 M⊙ at z = 0.08 − 0.33 from the Planck Sunyaev-Zel’dovich catalogue. Clusters without suitable radio data were observed with the Giant Metrewave Radio Telescope and/or the Jansky Very Large Array to complete the information about the possible presence of diffuse emission. We used archival Chandra X-ray data to derive information on the clusters’ dynamical states. Results. This observational campaign led to the detection of several cluster-scale diffuse radio sources and candidates that deserve future follow-up observations. Here we summarise their properties and add information resulting from our new observations. For the clusters where we did not detect any hint of diffuse emission, we derived new upper limits to their diffuse flux. Conclusions. We have built the largest mass-selected (> 80% complete in mass) sample of galaxy clusters with deep radio observations available to date. The statistical analysis of the sample, which includes the connection between radio halos and cluster mergers, the radio power – mass correlation, and the occurrence of radio halos as a function of the cluster mass, will be presented in Paper II.

A LOFAR-uGMRT spectral index study of distant radio halos

“…In support of this scenario, radio halos are preferentially detected in dynamically disturbed clusters (e.g. Cassano et al 2013;Cuciti et al 2021b). An additional channel may arise from protonproton collisions, which generate secondary electrons that then emit synchrotron radiation (e.g.…”

“…In those systems with no detection of diffuse emission in the uGMRT images, we derived upper limits injecting mock radio halos directly in the original visibilities (e.g. Venturi et al 2007Venturi et al , 2008Kale et al 2015;Cuciti et al 2021b). Following Bonafede et al (2017), we modelled the radio halo brightness with a symmetric exponential profile, I(r) = I 0 exp(−r/r e ), and including power spectrum fluctuations of the type P (Λ) ∝ Λ n (with Λ being the spatial scale of the fluctuations and n = 11/3; see also Govoni et al 2005Govoni et al , 2006Bonafede et al 2009).…”

“…In the exponential model, I 0 is the central brightness and r e the e-folding radius which we assumed to be one-third of the halo radius, R H 2 , (Murgia et al 2009). We injected the mock halo in a region close to the cluster 3 that is free of compact sources in order to avoid the inclusion of possible halo residuals which would bias the upper limits to lower levels (Bonafede et al 2017;Osinga et al 2021;Cuciti et al 2021b). We assumed different I 0 levels, starting from σ rms √ N beam and increasing this value until positive residuals were visible in the image.…”

“…We assumed different I 0 levels, starting from σ rms √ N beam and increasing this value until positive residuals were visible in the image. We define these positive residuals as an upper limit when the largest linear scale above the 2σ rms level is about 2r e (Cuciti et al 2021b). We measured the flux density directly from the image, corresponding to ∼ 80 − 90% of the injected flux.…”

Properties of Polarized Synchrotron Emission from Fluctuation Dynamo Action—II. Effects of Turbulence Driving in the ICM and Beam Smoothing

“…It is clear that observations with a telescope smears the filamentary features and significantly reduces the intensity contrast. Interestingly, radio continuum observations reveal that the emission from halos of galaxy clusters are spatially smooth [43], devoid of the filamentary structures seen at the native resolution of the simulations. Hence, it is important to assess the extent to which telescope beams smear out such structures, and whether they can be discerned with respect to the diffuse background emission.…”