Annu. Rev. Astron. Astrophys. 2002. 40: 643-680 Copyright © 2002 by Annual Reviews. All rights reserved

6. SUMMARY

The SZE is emerging as a powerful tool for cosmology. Over the last several years, detection of the SZE toward massive galaxy clusters has become routine, as has high quality imaging at moderate angular resolution of order an arcminute. Measurements of the effect already have been used to place interesting constraints on the Hubble constant and, through measurements of cluster gas mass fractions, the matter density of the universe, _M.

The next step is to exploit the redshift independence of the SZE signal to conduct blind surveys for galaxy clusters. The limit for such a survey is essentially a mass limit that is remarkably uniform with redshift. The cluster catalog from such a unbiased survey could be used to greatly increase the precision and redshift range of present SZE constraints on the Hubble constant and _M, and could, for example, allow D_A(z) to be determined to high redshift (z ~ 2).

The most powerful use of the SZE for cosmology will be the measurement of the evolution of the abundance of galaxy clusters. SZE surveys are ideally suited for this since they are able to probe the abundance at high redshift as easily as the local universe. The evolution of the abundance of galaxy clusters is a sensitive probe of cosmology. For example, the yields from a deep SZE survey covering only ten square degrees would be able to place interesting constraints on _M, , and ₈.

A generic prediction of inflation is that the primordial density fluctuations should be Gaussian. Non-Gaussianity in the form of an excess of high mass clusters should be readily apparent, especially at high redshift, from SZE survey yields. SZE cluster surveys will therefore probe both the structure formation history of the universe and the nature of the primordial fluctuations. In this way, cluster surveys are emerging as the next serious test of the cold dark matter paradigm.

Current SZE observations, while routine, require substantial integration time to secure a detection; a prohibitively long time would be required to conduct blind surveys over a large region of sky with the instruments now available. However, the next generation of instruments now being built or planned will be substantially faster. Dedicated interferometric arrays being built will be able to conduct deep SZE surveys over tens of square degrees. Heterogeneous arrays, such as the SZA combined with the OVRO array, will also allow detailed high resolution follow up SZE observations of the resulting cluster catalog.

A dedicated, low noise, single dish telescope with ~ 1' resolution, equipped with a next generation, large format bolometric array receiver (~ 1000 elements) and operating from a superb site would be able to conduct a deep SZE survey over thousands of square degrees. The statistics provided by the yields from such a large survey (~ 10⁴ clusters) in the absence of systematic effects and assuming redshifts are known would be sufficient to determine precise constraints on _M, , ₈, and even set meaningful constraints on the equation of state of the dark energy.

The possible systematics that could affect the yields of SZE surveys are presently too large to realize the full potential of a deep SZE survey covering thousands of square degrees. The systematics include, for example, the uncertainties on the survey mass detection limit due to unknown cluster structure and cluster gas evolution, as well as the uncertainties in the theoretical mapping between the initial density field and the number density of clusters of a given mass as a function of redshift, i.e., the mass function.

These systematics can begin to be addressed through detailed follow-up observations of a moderate area SZE survey (tens of square degrees). High resolution SZE, X-ray, and weak lensing observations will provide insights into evolution and structure of the cluster gas. Numerical simulations directly compared and normalized to the SZE yields should provide the necessary improvement in our understanding of the mass function.

It is not unreasonable to consider the possibility of a space-based telescope operating at centimeter through submillimeter wavelengths with high angular resolution (< 1 arcminute) and good spectral coverage. For studies of the SZE, this would allow simultaneous determinations of electron column densities, temperatures, and peculiar velocities of galaxy clusters. Such a satellite would make detailed images of the cosmic microwave background, while also providing important information on the high frequency behavior of radio point sources and the low frequency behavior of dusty extragalactic submillimeter sources. The upcoming Planck Surveyor satellite is a first step in this direction; it should provide an SZE all-sky survey although at moderate, ~ 5 arcminute, resolution. Such a survey should find on the order of 10⁴ - 10⁵ clusters, most of them at redshift z < 1.

We can look forward to the SZE emerging further as a unique and powerful tool in cosmology over the next several years as the next generation of SZE instruments come online and SZE surveys become a reality.

ACKNOWLEDGEMENTS

We thank M. Joy and W. Holzapfel for their considerable input to this review and W. Hu, S. LaRoque, A. Miller, J. Mohr, and D. Nagai for their comments on the manuscript. We also thank M. White and C. Pryke for assistance with Figure 3. This work was supported in part by NASA LTSA account NAG5-7986 and NSF account AST-0096913. JEC also acknowledges support from the David and Lucile Packard Foundation and the McDonnell Foundation. EDR acknowledges support from a NASA GSRP fellowship (NGT5-50173) and a Chandra Fellowship (PF1-20020).