Abstract
THE Uhuru (ref. 1) and the Ariel V (ref. 27) satellites have shown that many rich clusters are powerful sources of X rays. This emission has been interpreted either as thermal bremsstrahlung of very hot gas filling the clusters3,4, or as inverse Compton scattering of photons by relativistic electrons5,6. Spectral evidence2 has begun to favour a thermal origin for this radiation, implying the existence of large amounts of hot gas. Such gas would have a profound influence upon the dynamics of radio sources, and its mass might well be a substantial fraction of that of the whole cluster. Hot gas may therefore be a major constituent of the Universe, so that independent confirmation of its existence is extremely important. We present here observations of small diminutions in the cosmic microwave background radiation in the directions of several rich clusters of galaxies. This confirms the existence of large amounts of very hot gas in these clusters and indicates that their X radiation is thermal bremsstrahlung and not inverse Compton emission.
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