LOFAR discovery of a radio halo in the high-redshift galaxy cluster PSZ2 G099.86+58.45

In this Letter, we report the discovery of a radio halo in the high-redshift galaxy cluster PSZ2 G099.86+58.45 (z = 0.616) with the LOw Frequency ARray (LOFAR) at 120-168 MHz. This is one of the most distant radio halos discovered so far. The diffuse emission extends over ∼1 Mpc and has a morphology similar to that of the X-ray emission as revealed by XMM-Newton data. The halo is very faint at higher frequencies and is barely detected by follow-up 1-2 GHz Karl G. Jansky Very Large Array observations, which enable us to constrain the radio spectral index to be α ≲ 1.5-1.6, i.e., with properties between canonical and ultra-steep spectrum radio halos. Radio halos are currently explained as synchrotron radiation from relativistic electrons that are re-accelerated in the intracluster medium by turbulence driven by energetic mergers. We show that in such a framework radio halos are expected to be relatively common at ∼150 MHz (∼30%-60%) in clusters with mass and redshift similar to PSZ2 G099.86+58.45; however, at least two-thirds of these radio halos should have a steep spectrum and thus be very faint above ∼1 GHz frequencies. Furthermore, because the luminosity of radio halos at high redshift depends strongly on the magnetic field strength in the hosting clusters, future LOFAR observations will also provide vital information on the origin and amplification of magnetic fields in galaxy clusters.© 2019. The American Astronomical Society. All rights reserved.
M.S. acknowledges financial contribution from ASI-INAF n.2017-14-H.0. The Leiden LOFAR team acknowledge support from the European Research Council under the FP/2007-2013/ERC Advanced Grant NEWCLUSTERS-321271. R.J.v.W. acknowledges support from the VIDI research program, project number 639.042.729, which is financed by the NWO. A.B. acknowledges financial support from the ERC-Stg DRANOEL, no 714245, and from the MIUR grant FARE SMS. L.I. acknowledges support from funding associated with Juan de la Cierva Incorporacion fellowship IJCI-2016-30940. A.P.M. acknowledge support from the NWO/DOME/IBM program >Big Bang Big Data: Innovating ICT as a Driver For Astronomy,> project #628.002.001. G.B. acknowledges partial support from ASI/INAF n.2017-14-H.O