An image of the Sunyaev-Zel'dovich effect

THE Sunyaev-Zel'dovich effect¹ is a distortion imposed on the 2.7-K cosmic microwave background radiation when the microwave photons are scattered by the hot gas in galaxy clusters. At radio wavelengths it appears as a decrement (of about 0.5 mK) in the brightness temperature of the background radiation. Measurements of this effect can provide information about the physics of galaxy clusters, and can also potentially be used to determine the Hubble constant, by constraining the size of the cluster along the line of sight. Successful detections have been made with singledish radio telescopes ^2,3, from which one-dimensional profiles of the temperature decrement are constructed. But these observations are susceptible to several systematic errors, such as corruption of the measured signal by radio sources located close to the line of sight. To circumvent these problems, we have obtained a two-dimensional image, using an interferometric (rather than single-dish) approach, of structure in the microwave background in the vicinity of the galaxy cluster Abell 2218. Our measurements, combined with X-ray observations of the scattering gas in the same cluster, support a low value of the Hubble constant.