Radio halos in nearby (z < 0.4) clusters of galaxies

¹ Dipartimento di Astronomia, via Ranzani 1, 40127 Bologna, Italy e-mail: ggiovann@ira.inaf.it
² Istituto di Radioastronomia-INAF, via P.Gobetti 101, 40129 Bologna, Italy
³ Osservatorio Astronomico di Cagliari - INAF, Strada 54, Loc. Poggio dei Pini, 09012 Capoterra (Ca), Italy

Received: 9 June 2009
Accepted: 4 September 2009

Abstract

Context. The intra-cluster medium is characterized by thermal emission, and by the presence of large scale magnetic fields. In some clusters of galaxies, a diffuse non-thermal emission is also present, located at the cluster center and called radio halo. These sources indicate the existence of relativistic particles and magnetic fields in the cluster volume.

Aims. In this paper we collect data on all known nearby cluster radio halos (), to discuss their statistical properties and to investigate their origin.

Methods. We searched for published data on radio halos and reduced new and archive VLA data to increase the number of known radio halos.

Results. We present data on 31 radio halos, 1 new relic source, and 1 giant filament. We note the discovery of a small size diffuse radio emission in a cluster (A1213) with very low X-ray luminosity. Among the statistical results, we confirm the correlation between the average halo radio spectral index and the cluster temperature. We also discuss the high percentage of clusters where both a relic and a radio halo is present.

Conclusions. The sample of radio halos discussed here represents the population of radio halos observable with current radio telescopes. A new telescope generation is necessary for a more detailed multifrequency study, and to investigate the possible existence of a population of radio halos with different properties.