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 toward galaxy clusters due to its Compton scattering by electrons of the hot intergalactic gas. When mapping the background fluctuations at frequencies below \({\lesssim}800\) MHz, this effect gives rise to a radio source at the location of the cluster. At higher frequencies, where the microwave (relic) radiation dominates in the cosmic background, a ‘‘negative’’ source (a ‘‘shadow’’ on the map of background fluctuations) is observed at the location of the cluster due to the transfer of some of the relic photons upward along the frequency axis upon their scattering (into the range \(\nu\gtrsim 217\) GHz; Sunyaev and Zeldovich 1970, 1972). We have computed the spectra of the expected radio background distortions for various parameters of clusters and show that in many cases in the wide frequency range \(30\ \textrm{MHz}\lesssim\nu\lesssim 3\ {\textrm{GHz}}\) the measurement of the distortions will be hindered by the intrinsic thermal (bremsstrahlung) radiation from the intergalactic gas and the scattered radio emission from cluster galaxies associated with their past activity, including the synchrotron radiation from ejected relativistic electrons. Below \({\sim}20\) MHz the scattering effect always dominates over the thermal gas radiation due to the general increase in the intensity of the cosmic radio background, but highly accurate measurements at such frequencies become difficult. Below \({\sim}5\) MHz the effect is suppressed by the induced scattering. We have found the frequency ranges that are optimal for searching for and measuring the Compton radio background excess. We show that hot (\(kT_{\textrm{e}}\gtrsim 8\ {\textrm{keV}}\)) clusters at high (\(z\gtrsim 0.5\)) redshifts are most promising for its observation. Because of the strong concentration of the bremsstrahlung to the cluster center, the peripheral observations of the Compton excess must be more preferable than the central ones. Moreover, owing to the thermal radiation of the gas and its concentration to the center, the above-noted transition from the ‘‘negative’’ source on the map of background fluctuations to the ‘‘positive’’ one when moving downward along the frequency axis must occur not gradually but through the stage of a ‘‘hybrid source’’—the appearance of a bright spot surrounded by a dark ring. This form of the source in projection is explained by its unusual three-dimensional shape in the form of a narrow radio bremsstrahlung peak rising from the center of a wide deep hole associated with the Compton scattering of the cosmic microwave background. The scattered radiation from an active central cluster galaxy in the past can amplify the effect. An analogous ‘‘hybrid source’’ also appears on the map of background fluctuations near a frequency of \(217.5\) GHz—when passing from the deficit of the cosmic microwave background to its excess (due to the scattered photons). The unusual shape of the source is again associated with the thermal gas radiation. Simultaneous measurements of the radio bremsstrahlung flux from the gas and the amplitude of the distortions due to the radio and cosmic microwave background scattering will allow the most important cluster parameters to be determined.