Observations of the Sunyaev-Zeldovich Effect Towards ROSAT Clusters with SEST and the Italian Double-Channel Photometer

A double-channel (1.2 and 2 mm) photometer has been installed at the focus of the SEST antenna to perform measurements of the Sunyaev-Zeldovich effect towards X-ray ROSAT clusters. Here we report the performance of the instrument and the first results obtained during the August-September 1994 observing run. The Sunyaev-Zeldovich effect (Sunyaev and Zeldovich, 1972) is a shift of the Cosmic Microwave Background spectrum by the inverse Compton scattering of microwave photons by the hot electron gas present in rich clusters of galaxies. The resulting cluster signal shows a decrement at wavelengths longer than 1.4 mm and an enhancement at shorter ones relative to its planckian value. After many attempts to detect this effect (see e.g. the review paper Birkinshaw, 1990) radio observations seem finally to show the expected decrement at centimetre wavelengths towards A2218, A665, 0016+16 and A773 (Birkinshaw, 1991; Klein et aI., 1991; Jones et aI., 1993; Grainge et aI., 1993) and more recently even at 2.2 mm towards A2163 (Wilbanks et aI., 1994). However, a more definitive detection of the S-Z effect requires the measurement of both the decrement and the enhancement. In order to minimize systematic errors and to eliminate spurious signals, simultaneous detections of the decrement and enhancement are mandatory. The Italian group of the III University of Rome have therefore built a photometer with two channels centred at 1.2 and 2 mm to feed the O.A.S.1. (Osservatorio Antartico Submillimetrico Infrarosso) telescope installed at the Italian base in Antarctica (Dall'Oglio et aI., 1992). This photometer has been adapted to be placed at the focus of the SEST antenna in Chile.