Detection of intergalactic gas in distant, rich clusters

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WE have detected a reduction in intensity or ‘cooling’ of the cosmic microwave background as this radiation passes through several rich clusters of galaxies. These observations imply the presence of substantial quantities of hot intergalactic gas in the clusters observed. The ‘cooling’ effect, due to inverse Compton scattering of the microwave photons by hot intergalactic gas in the clusters, was predicted earlier by Sunyaev and Zel'dovich1. The fractional change in the intensity of the microwave background is given by where x is related to the observing frequency by x = hv/kTr, with Tr the temperature of the microwave background, taken to be 2.7 K; τ is the total optical depth for Thomson scattering through a cluster; and me and Te are the electron mass and electron temperature of the hot gas. Gull and Northover2, observing at λ = 3 cm, have recently reported evidence for this effect in several clusters. We have tried to confirm their work at somewhat higher sensitivity, and at a shorter wavelength. A shorter wavelength was chosen to lessen interference and confusion by radio sources in or near the clusters observed. Our results on eight clusters which are Uhuru X-ray sources will be reported elsewhere3. In general, we were able to set 2σ limits of ≲4 × 10−4 K on the reduction in temperature of the cosmic background; in some cases these limits lie below the results reported by Gull and Northover. We do see statistically significant evidence for the ‘cooling’ effect in three clusters, however—all of which are in Abell4 richness class 4. In the case of one of the three, Abell 2218, our results confirm the measurement of ΔT = − 1.94 ± 0.54 mK by Gull and Northover2. Here we call attention to the apparent strong correlation between the richness class and the detectability of the inverse Compton ‘cooling’. These results may be of interest to those constructing models for the intergalactic plasma in clusters5–11, especially when the results are combined with the more sensitive X-ray observations we can expect if the new HEAO satellites are successful.