4.2 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 (Sunyaev & Zel'dovich 1969). Given a spatial 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. 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. Some of the main uncertainties result from 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 ~ 40-80 km/sec/Mpc (e.g., Birkinshaw 1998). 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 are improving enormously.

Carlstrom, (this meeting) presented exquisite new measurements of the Sunyaev-Zel'dovich decrement, measured in two dimensions with an interferometer, for a number of nearby clusters. He also highlighted the imminent advances on the horizon for X-ray imaging using NASA's soon-to-be launched Chandra X-ray Observatory (the satellite formerly known as AXAF). There is promise of a significant increase in accuracy for this method in the near future.