5.3 Sunyaev Zel'dovich Effect and X-Ray Measurements
The inverse-Compton scattering of photons from the cosmic microwave background off of hot electrons in the X-ray gas of rich clusters results in a measurable decrement in the microwave background spectrum known as the Sunyaev-Zel'dovich (SZ) effect (Zel'dovich and Sunyaev 1969). Given a spatial (preferably 2-dimensional) distribution of the SZ effect and a high-resolution X-ray map, the density and temperature distributions of the hot gas can be obtained; the mean electron temperature can be obtained from an X-ray spectrum. An estimate of H0 can be made based on the definitions of the angular-diameter and luminosity distances. The method makes use of the fact that the X-ray flux is distance-dependent, whereas the Sunyaev-Zel'dovich decrement in the temperature is not.
Once again, the advantages of this method are that it can be applied at large distances and, in principle, it has a straightforward physical basis. As discussed in Section 5.1, some of the main uncertainties with this method are due to potential clumpiness of the gas (which would result in reducing H0), projection effects (if the clusters observed are prolate, H0 could be larger), the assumption of hydrostatic equilibrium, details of the models for the gas and electron densities, and potential contamination from point sources.
To date, a range of values of H0 have been published based on this method ranging from ~ 25-80 km/sec/Mpc (e.g., McHardy et al. 1990; Birkinshaw & Hughes 1994; Rephaeli 1995; Herbig, Lawrence & Readhead 1995). The uncertainties are still large, but as more and more clusters are observed, higher-resolution (2D) maps of the decrement, and X-ray maps and spectra become available, the prospects for this method will continue to improve. At this conference, Carlstrom reported on a new extensive survey of lenses being undertaken both at Hat Creek and the Owens Valley Radio Observatory. X-ray images are being obtained with ROSAT and X-ray spectra with ASCA.