A LOFAR-uGMRT spectral index study of distant radio halos

¹ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
e-mail: digennaro@strw.leidenuniv.nl
² Istituto Nazionale di Astrofisica-Istituto di Radioastronomia, Bologna Via Gobetti 101, 40129 Bologna, Italy
³ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁴ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
⁵ GEPI and USN, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
⁶ Centre for Radio Astronomy Techniques and Technologies, Department of Physics and Electronics, Rhodes University, Grahamstown 6140, South Africa

Received: 9 June 2021
Accepted: 3 August 2021

Abstract

Context. Radio halos are megaparsec-scale diffuse radio sources mostly located at the centres of merging galaxy clusters. The common mechanism invoked to explain their origin is the re-acceleration of relativistic particles caused by large-scale turbulence.

Aims. Current re-acceleration models predict that a significant number of halos at high redshift should be characterised by very steep spectra (α < −1.5) because of increasing inverse Compton energy losses. In this paper, we investigate the spectral index properties of a sample of nine clusters selected from the second Planck Sunyaev-Zel’dovich catalogue showing diffuse radio emission with the Low Frequency Array (LOFAR) in the 120–168 MHz band. This is the first time that radio halos discovered at low frequencies are followed up at higher frequencies.

Methods. We analysed upgraded Giant Metrewave Radio Telescope (uGMRT) observations in Bands 3 and 4, that is, 250–500 and 550–900 MHz respectively. These observations were combined with existing LOFAR data to obtain information on the spectral properties of the diffuse radio emission.

Results. We find diffuse radio emission in the uGMRT observations for five of the nine high-z radio halos previously discovered with LOFAR. For those, we measure spectral indices in the range of −1 to −1.4. For the uGMRT non-detections, we estimated that the halos should have a spectral index steeper than −1.5. We also confirm the presence of one candidate relic.

Conclusions. Despite the small number of clusters, we find evidence that about half of the massive and merging clusters at high redshift host radio halos with a very steep spectrum. This is in line with theoretical predictions, although larger statistical samples are necessary to test models.