Radio halos in future surveys in the radio continuum

¹ INAF – Istituto di Radioastronomia, via P. Gobetti 101, 40129 Bologna, Italy
e-mail: rcassano@ira.inaf.it
² School of Chemical & Physical Sciences, Victoria University of Wellington, Wellington 6140, New Zealand
³ CSIRO Astronomy & Space Science, NSW 1710 Epping, Australia
⁴ Leiden Observatory, Leiden University, Oort Gebouw, PO Box 9513, 2300 RA Leiden, The Netherlands
⁵ Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany

Received: 13 July 2012
Accepted: 2 October 2012

Abstract

Aims. Giant radio halos are Mpc-scale synchrotron sources detected in a significant fraction of massive and merging galaxy clusters. The statistical properties of radio halos can be used to discriminate among various models for the origin of non-thermal particles in galaxy clusters. Therefore, theoretical predictions are important as new radio telescopes are about to begin to survey the sky at low and high frequencies with unprecedented sensitivity.

Methods. We carry out Monte Carlo simulations to model the formation and evolution of radio halos in a cosmological framework and extend previous calculations based on the hypothesis of turbulent-acceleration. We adopt a phenomenological approach by assuming that radio halos are either generated in turbulent merging clusters, or are purely hadronic sources generated in more relaxed clusters, “off-state” halos.

Results. The models predict that the luminosity function of radio halos at high radio luminosities is dominated by the contribution of halos generated in turbulent clusters. The generation of these halos becomes less efficient in less massive systems causing a flattening of the luminosity function at lower radio luminosities, as also pointed out in previous studies. However, we find that potentially this can be more than compensated for by the intervening contribution of “off-state” halos that dominate at lower radio luminosities. We derive the expected number of halos to explore the potential of the EMU+WODAN surveys that will be carried out with ASKAP and Aperitif, respectively, in the near future. By restricting to clusters at redshifts  ≤ 0.6, we show that the planned EMU+WODAN surveys at 1.4 GHz have the potential to detect up to about 200 new radio halos, increasing their number by one order of magnitude. A fraction of these sources will be “off-state” halos that should be found at flux level f_1.4  ≤  10 mJy, presently accessible only to deep pointed observations. We also explore the synergy between surveys at different radio frequencies, the Tier 1 LOFAR survey at 150 MHz and the EMU+WODAN surveys at 1.4 GHz. We predict a larger number of radio halos in the LOFAR survey due to the high LOFAR sensitivity, but also due to the existence of halos with very steep spectrum that glow up preferentially at lower frequencies. These halos are only predicted in the framework of turbulent re-acceleration models and should not have counterparts in the EMU+WODAN surveys, thus the combination of the two surveys will test theoretical models.