The Sunyaev-zel’dovich Effect

The Sunyaev-Zel’dovich effect causes a change in the apparent brightness of the Cosmic Microwave Background Radiation towards a cluster of galaxies or any other reservoir of hot plasma. Measurements of the effect provide distinctly different information about cluster properties than X-ray imaging data, while combining X-ray and Sunyaev-Zel’dovich effect data leads to new insights into the structures of cluster atmospheres. The effect is redshift-independent, and so provides a unique probe of the structure of the Universe on the largest scales. The present review discusses the theory of the Sunyaev-Zel’dovich effect and collects published results for many clusters, presents the overall conclusions that may be drawn from the detections so far, and discusses the prospects for future research on the Sunyaev-Zel’dovich effects. Subject headings: Sunyaev-Zel’dovich Effect; Clusters; Microwave Background Radiation 1. Astrophysical Context Compton scattering is one of the major physical processes that couples matter and radiation. Its importance is often stressed in highly relativistic environments where large energy transfers occur: for example, in the synchrotron self-Compton process that may be responsible for much of the X-radiation from active galactic nuclei (e.g., Fabian et al. 1986). However, the Compton process also has observable consequences in low-energy environments, where small energy transfers occur. The Sunyaev-Zel’dovich effect, which arises from the scattering of electrons in clusters of galaxies on the cosmic microwave Also Smithsonian Institution Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138, USA