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

The Sunyaev-Zel'dovich Effect (SZE) offers a unique and powerful
observational tool for cosmology. Recently, there has been
considerable progress in detecting and imaging the SZE. Efforts over
the first two decades after the SZE was first proposed in 1970
(Sunyaev &
Zel'dovich, 1970,
1972)
yielded few reliable detections. Over the
last decade, new detectors and observing techniques have allowed high
quality detections and images of the effect for more than 50 clusters
with redshifts as high as one. The next generation of SZE instruments
that are now being built or planned will be orders of magnitude more
efficient. Entering the fourth decade of SZE observations, we are now
in position to exploit fully the power of the SZE, by obtaining
detailed images of a set of clusters to understand the intra-cluster
medium (ICM), by obtaining large SZE samples of clusters to determine
statistically robust estimates of the cosmological parameters and,
most importantly, by conducting large untargeted SZE surveys to probe
the high redshift universe. These surveys will provide a direct view
of the growth of large scale structure and will provide large catalogs
of clusters that extend past *z* ~ 2 with remarkably uniform
selection functions.

The physics of the SZE has been covered well in previous reviews (Birkinshaw, 1999, Rephaeli, 1995, Sunyaev & Zel'dovich, 1980a), with Birkinshaw (1999) and Carlstrom et al (2000) providing recent reviews of the observations. In this review, we look to the near future, using recent observations as a guide to what we can expect.

The SZE is best known for allowing the determination of cosmological
parameters when combined with other observational diagnostics of
clusters of galaxies such as X-ray emission from the intracluster gas,
weak and strong lensing by the cluster potential, and optical galaxy
velocity dispersion measurements. For example, cluster distances have
been determined from the analysis of SZE and X-ray data, providing
independent estimates of the Hubble constant. A large homogeneous
sample of galaxy clusters extending to high redshift should allow a
precise measure of this number, as well as a measure of the angular
diameter distance relation to high redshift where it is highly
sensitive to cosmological parameters. Similarly, the SZE and X-ray
measurements will allow tight constraints on cluster gas mass
fractions which can be used to estimate
_{M}
assuming the
composition of clusters represents a fair sample of the universal
composition. The observed redshift dependence of the gas mass fraction
can also be used to constrain cosmological parameters as well as test
speculative theories of dark matter decay.

The most unique and powerful cosmological tool provided by the
exploitation of the SZE will likely be the direct measurement of the
evolution of the number density of galaxy clusters by deep, large
scale SZE surveys. The redshift evolution of the cluster density is
critically dependent on the underlying cosmology, and in principle can
be used to determine the equation of state of the dark energy.
SZE observations are particularly well suited for deep surveys because
the important parameter that sets the detection limit for such a
survey is the mass of the cluster;
SZE surveys will be able to detect all clusters above a mass
limit independent of the redshift of the clusters. This remarkable
property of SZE surveys is due to the fact that the SZE is a
distortion of the cosmic microwave background (CMB) spectrum. While
the CMB suffers cosmological dimming with redshift, the ratio of the
magnitude of the SZE to the CMB does not; it is a direct, redshift
independent measurement of the ICM column density weighted by
temperature, i.e., the pressure integrated along the line of
sight. The total SZE flux detected will be proportional to the total
temperature-weighted mass (total integrated pressure) and, of course,
inversely proportional to the square of the angular diameter
distance. Adopting a reasonable cosmology and accounting for the
increase in the universal matter density with redshift, the mass limit
for a given SZE survey flux sensitivity is not expected to change more
than a factor of ~ 2 - 3 for any clusters with *z* > 0.05.

SZE surveys therefore offer an ideal tool for determining the cluster density evolution. Analyses of even a modest survey covering ~ 10 square degrees will provide interesting constraints on the matter density of the universe. The precision with which cosmological constraints can be extracted from much larger surveys, however, will be limited by systematics due to our insufficient understanding of the structure of clusters, their gas properties and evolution.

Insights into the structure of clusters will be provided by high
resolution SZE observations, especially when combined with other
measurements of the clusters. Fortunately, many of the cluster
properties derived directly from observational data can be determined
in several different ways. For example, the gas mass fraction can be
determined by various combinations of SZE, X-ray, and lensing
observations. The electron temperature, a direct measure of a
cluster's mass, can be measured directly through X-ray spectroscopy,
or determined through the analysis of various combinations of X-ray,
SZE, and lensing observations. Several of the desired properties of
clusters are therefore over-constrained by observation, providing
critical insights to our understanding of clusters, and critical tests
of current models for the formation and evolution of galaxy clusters.
With improved sensitivity, better angular resolution, and sources out
to *z* ~ 2, the next generation of SZE observations will provide a
good view of galaxy cluster structure and evolution. This will allow,
in principle, the dependence of the cluster yields from large SZE
surveys on the underlying cosmology to be separated from the
dependence of the yields on cluster structure and evolution.

We outline the properties of the SZE in the next section and provide
an overview of the current state of the observations in
Section 3. This is followed in
Section 4
by predictions for the expected yields of upcoming SZE surveys. In
Section 5, we provide an overview of the
cosmological tests which will be possible with catalogs of SZE-selected
clusters. This is followed by a discussion of backgrounds,
foregrounds, contaminants, and theoretical uncertainties that could
adversely affect cosmological studies with the SZE and a discussion of
observations which could reduce or eliminate these concerns.
Throughout the paper, *h* is used to parametrize the Hubble constant
by *H*_{0} = 100*h* km s^{-1}
Mpc^{-1}, and
_{M} and
_{} are
the matter density and vacuum energy density, respectively, in units of
the critical density.