Increase in the Brightness of the Cosmic Radio Background toward Galaxy Clusters

Abstract

We explore the possibility of detecting the excess of the cosmic radio background (CRB) toward galaxy clusters due to its Compton scattering by electrons of the hot intergalactic gas. When mapping the background fluctuations at frequencies < 800 MHz, this effect gives rise to a radio source at the cluster location. At higher frequencies, where the microwave (CMB or relic) radiation dominates in the background, a 'negative' source is observed at this location due to the transfer of some of the relic photons upward along the frequency axis (into the range > 217 GHz) upon their scattering. We have computed the spectra of the expected CRB distortions for various clusters and show that in many cases in the range 30 MHz - 3 GHz their measurement will be hindered by the thermal bremsstrahlung from the intergalactic gas and the scattered radio emission from cluster galaxies associated with their past activity (and the synchrotron radiation from relativistic electrons). Below 20 MHz the scattering effect always dominates over the thermal gas radiation due to the general increase in the CRB intensity. We have found the frequency ranges that are optimal for searching for the Compton CRB excess and show that hot (kTe > 8 keV) clusters at high (z > 0.5) redshifts are most promising for its observation. Because of the strong concentration of the bremsstrahlung to the cluster center, the peripheral observations of this excess must be more preferable than the central ones. The transition from the 'negative' source on the map of background fluctuations to the 'positive' one must occur not gradually but through the stage of a 'hybrid source' (a bright spot surrounded by a dark ring). This shape of the source in projection is explained by its unusual three-dimensional shape (a narrow bremsstrahlung peak rising from the center of a wide deep hole associated with the Compton scattering of the CMB).