First Measurement of Cluster Temperature Using the Thermal Sunyaev-Zel’dovich Effect

We discuss a new method of finding cluster temperatures that is independent of distance and is therefore very useful for distant clusters. The hot gas of electrons in clusters of galaxies scatters and distorts the cosmic microwave background radiation in a well-determined way. This Sunyaev-Zel'dovich (SZ) effect is a useful tool for extracting information about clusters, such as their peculiar radial velocity and optical depth. Here we show how the temperature of the cluster can be inferred from the SZ effect, in principle, without use of X-ray data. We use recent millimeter observations of Abell 2163 to determine for the first time a cluster temperature using SZ observations only. The result, T e = 26 keV at 68% confidence level (at 95% confidence level we find T > 1.5 keV), is in reasonable agreement with the X-ray results, T e = 12.4 keV.