Annu. Rev. Astron. Astrophys. 1992. 30: 653-703
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4.5 The Sunyaev-Zel'dovich Effect

The Sunyaev-Zel'dovich effect was last reviewed in these pages in 1980 (Sunyaev & Zel'dovich 1980a). At that time there were no convincing detections of the Sunyaev-Zel'dovich effect, although a number of claims had been made which were not substantiated by further observations, or by other groups. The maximum decrements at centimeter wavelengths due to the Sunyaev-Zel'dovich effect expected from rich clusters of galaxies lie in the range ~ 100-1000 µK. Successful observations of the Sunyaev-Zel'dovich effect therefore depend on observations with sensitivity ~ 20 µK. Thus the same systematic effects which plague observations of intrinsic anisotropy complicate observations of the Sunyaev-Zel'dovich effect - atmospheric emission, ground spillover, receiver gain variations, etc.

Table 5. Recent measurements of the Sunyaev-Zel'dovich effect

Cluster thetaex a DeltaTRJ (mK) b DeltaT(0) (mk) c References

0016+16 1'.0 -0.48 ± 0.12 -0.78 ± 0.20 Uson 1987
-0.44 ± 0.10 -0.75 ± 0.17 Birkinshaw 1990
Abell 665 2'.0 -0.37 ± 0.14 -0.51 ± 0.19 Uson 1987
-0.30 ± 0.10 -0.44 ± 0.15 Birkinshaw 1990
Abell 2218 2'.1 -0.29 ± 0.24 -0.40 ± 0.33 Uson 1985
-0.35 ± 0.09 -0.51 ± 0.13 Birkinshaw 1990

a Angular size of X-ray core.
b Rayleigh-Jeans decrement at observed frequency.
c Equivalent decrement at nu = 0.

For the purposes of this review the Sunyaev-Zel'dovich effect is a foreground contamination, and we refer the interested reader to the reviews by Birkinshaw (1990, 1991) for more detailed accounts of the field. Birkinshaw (1991) lists all published attempts at Sunyaev-Zel'dovich effect measurements (see his Table 1). The results on the same clusters by different observers, and even of the same observers at different times, do not agree (see his Table 2). The most sensitive observations are those of Birkinshaw and his collaborators (Birkinshaw et al 1984, 1991, 1992; Birkinshaw 1990, 1991) and of Uson (1985, 1987). In their observations since 1984, the results from these two groups have been consistent on three rich clusters - 0016+16, Abell 665 and Abell 2218, as shown in Table 5. Birkinshaw's results for different offset positions in these three clusters are shown in Figure 8. It is clear that the Sunyaev-Zel'dovich effect has indeed been detected in these three clusters.

Figure
 8
Figure 8. Results of observations of the Sunyaev Zel'dovich effect in three clusters at 20 GHz by Birkinshaw et al (1992). The results are plotted as a function of the offsets in declinations from the nominal cluster centers. Crosses represent the data with ± 1sigma errors deduced from the full dataset, with the widths of the crosses equal to the FWHM of the OVRO 40 meter telescope beam. Thicker extensions to the erorr bars indicate increases in the errors implied by year-to-year discordance in the data. Boxes around each point represent the maximum estimated systematic errors for each point, due to uncertainties in confusion by discrete sources. The estimated level of the local zero in each scan is indicated by the dotted lines, which mark the ± 1sigma range of DeltaTzero. The peak decrement in Abell 665 occurs 2 arcminutes south of the center of the optical cluster, but coincides with the centroid of X-ray emission (Birkinshaw et al 1991).

Bond and Myers (1991a, b) have constructed a cold dark matter model using a Monte Carlo hierarchical peak simulation method. This gives them the three-dimensional distribution of galaxy clusters for various values of the biassing parameter, brho. They point out that clusters are rare events defined by the tail of a Gaussian probability distribution so that the mass function for clusters is very sensitive to the value of brho. They adopt plausible cluster parameters and hence calculate the X-ray emissivity of the clusters and also the expected Sunyaev Zel'dovich effect from their three-dimensional model. An example for the case of brho = 1 is shown in Figure 9. In this simulation the Sunyaev Zel'dovich effect map has been convolved with the 1'.8 beam of the 40 meter telescope of the Owens Valley Radio Observatory. For this map the average y- parameter is 1 x 10-6, and in the Rayleigh-Jeans regime deltatbar = -2ybar. The rms value of DeltaT/T is 6.3 x 10-6, about a factor of two below the present limits of isotropy observations, but well within the range of the sensitivity levels that we are seeking. The corresponding values for the map derived by Bond and Myers for brho = 1.4 are significantly lower than these.

Figure
 9
Figure 9. A simulated contour image, by Bond and Myers (1991a), of Sunyaev Zel'dovich effect sources in the cold dark matter model with biassing brho = 1. The original map has been convolved with a 1'.8 beam. The contour levels decrease by factors of two from the peak at DeltaT = -5 x 10-6(OmegaB / 0.05).

It is clear from these observations and simulations that conditions in at least some clusters are responsible for significant distortions in the microwave background radiation, and that the Sunyaev-Zel'dovich effect is must be taken into account when searching for intrinsic anisotropy in the microwave background radiation.

There are many aspects of the Sunyaev-Zel'dovich effect which make it a useful tool in cosmological studies (e.g. Sunyaev & Zel'dovich 1980a, b, Birkinshaw et al 1991), but perhaps the most important from the point of view of the present review is that the detection of a decrement in the direction of distant clusters demonstrates directly and unequivocally that the microwave background radiation does indeed originate at high redshift.

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