Detection of Intergalactic Gas in Distant Rich Clusters
Radio observations of three Abell clusters of galaxies of richness class 4 and distance class 6 at a wavelength of 9 mm are reported which reveal a reduction in intensity or a 'cooling' of the cosmic microwave background as this radiation passes through the clusters. The 'cooling' effect is attributed to inverse Compton scattering of the microwave photons by hot intergalactic gas in the clusters. The measured changes in the temperature of the microwave background are given along with their associated standard deviations of the mean, and approximate values of the proton density in the clusters are derived under the assumptions of a specific adiabatic model. An apparent strong correlation is noted between richness class and the detectability of the inverse Compton 'cooling'. It is suggested that the three rich clusters observed should be luminous X-ray sources.
(with G. Lake*) Detection of Intergalactic Gas in Distant Rich Clusters, Nature, 270, 502, 1977.