A LOFAR-uGMRT spectral index study of distant radio halos
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.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- October 2021
- DOI:
- 10.1051/0004-6361/202141510
- arXiv:
- arXiv:2108.10562
- Bibcode:
- 2021A&A...654A.166D
- Keywords:
-
- radiation mechanisms: non-thermal;
- galaxies: clusters: general;
- galaxies: clusters: intracluster medium;
- Astrophysics - Cosmology and Nongalactic Astrophysics;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 23 pages, 22 figures, 4 tables