Exploring the nature of radio relics and halos in galaxy clusters through GHz radio observations

Abstract

Clusters of galaxies are the largest gravitationally bound systems in the Universe. A fraction of them hosts diffuse Mpc-scale synchrotron sources (referred to as radio relics and radio halos) not related to any discrete cluster member, but rather to the diffuse medium. The emission from these sources helps in unambiguously prove the presence of weak magnetic fields and non-thermal plasma in the intra-cluster medium (ICM) of galaxy clusters, coexisting with the hot thermal component emitting X-rays. The radio emitting electrons, given their relatively short lifetime respect to the extent of these sources, must have undergone some form of acceleration. It is widely believed that they are connected to the most energetic phenomena in the Universe, mergers between clusters, during which shocks and turbulence may develop in the ICM leading eventually to the acceleration of particles through Fermi mechanisms. Both phenomena are, indeed, mostly observed in galaxy clusters with a disturbed dynamical state. However, the details of the acceleration mechanisms are still greatly debated. According to the proposed models, the currently known radio relics and halos are the most energetic cases, for which relatively high-frequency observations are necessary in order to test the models expectations. However, for most of them the highest studied frequency is 1.4 GHz. <br /> In this thesis we studied the properties of this kind of sources at frequencies > 1 GHz. We mainly focused on two clusters, Abell 1656 (best known as the Coma cluster) and Abell 2256, known to possess both a radio relic and a radio halo. We observed a wide field on the Coma cluster, covering both the peripheral relic and the central halo, with the Effelsberg 100-m telescope at 1400 MHz. Moreover, the Coma relic field and the Coma halo field have been covered separately with respectively a 3-pointing and a 9-pointing mosaics performed with the WSRT at 2273 MHz. We have observed the cluster Abell 2256 with the Effelsberg-100m Telescope at 2640 MHz and at 4850 MHz. The same field was observed with a 3-pointing mosaic performed with the WSRT at 2273 MHz. The observations were performed in full polarization mode.<br /> The Effelberg observations were carried out during an observational campaign of a small sample of galaxy clusters known to host diffuse non-thermal emission. We include preliminary results from the analysis of Effelsberg C-band data of the galaxy clusters Abell 0115 and Abell 2255. We highlight the importance of high-frequencies observations for the study of both the total intensity and the polarization properties of these sources. The connection of the radio properties observed with the properties of the thermal gas derived from X-ray and S-Z effect observations is also discussed.<br /> It is generally assumed that radio relics are observed in stationary conditions. In this case the widely accepted model for radio relics, the diffusive acceleration at merger shocks (DSA), predicts single power-law integrated spectra. However, we observe a departure from a pure power-law integrated spectrum at frequencies > 1.4 GHz for the radio relic in Abell 2256 that puts tension on the commonly assumed conditions for DSA. <br /> Moreover, theoretical studies have shown that in the case of strong shocks the DSA model predicts magnetic field compression and alignment with the shock front. At high frequencies, the observed polarized emission is less affected by Faraday effects and it is therefore a better indicator of the intrinsic properties of the sources. Possible alignment of the polarization vectors measured across the extent of radio relics can, therefore, be used to reconstruct the geometry of the shock passage in case of shock acceleration. We show that the polarization properties in the Abell 2256 relic are difficult to be reconciled with a single outgoing shock front, requiring a more complex scenario.<br /> We also show how the combination of interferometric and single-dish data in the Fourier domain (applied to the observations of the Coma cluster) allow us to overcome the "missing short spacings problem" suffered by interferometers at these frequencies, that makes them blind to very extended emission. The combination technique is, therefore, a promising tool to study the properties of radio relics and halos at GHz frequencies.