Elsevier

Physics Reports

Volume 310, Issues 2–3, March 1999, Pages 97-195
Physics Reports

The Sunyaev–Zel’dovich effect

https://doi.org/10.1016/S0370-1573(98)00080-5Get rights and content

Abstract

The Sunyaev–Zel’dovich effect causes a change in the apparent brightness of the cosmic microwave background radiation towards a cluster of galaxies or any other reservoir of hot plasma. Measurements of the effect provide distinctly different information about cluster properties than X-ray imaging data, while combining X-ray and Sunyaev–Zel’dovich effect data leads to new insights into the structures of cluster atmospheres. The effect is redshift-independent, and so provides a unique probe of the structure of the Universe on the largest scales. The present review discusses the theory of the Sunyaev–Zel’dovich effect and collects published results for many clusters, presents the overall conclusions that may be drawn from the detections so far, and discusses the prospects for future research on the Sunyaev–Zel’dovich effects.

Section snippets

Astrophysical context

Compton scattering is one of the major physical processes that couples matter and radiation. Its importance is often stressed in highly relativistic environments where large energy transfers occur: for example, in the synchrotron self-Compton process that may be responsible for much of the X-radiation from active galactic nuclei (e.g., Fabian et al., 1986). However, the Compton process also has observable consequences in low-energy environments, where small energy transfers occur. The

Radiation basics

Although the CMBR is close to being an isotropic and thermal radiation background with simple spectral and angular distributions, it is useful to recall the formalism needed to deal with a general radiation field, since the details of the small perturbations have great physical significance. The notation used here is similar to that of Shu (1991), which may be consulted for more detailed descriptions of the quantities employed.

The state of a radiation field can be described by distribution

Inverse-Compton scattering

The theoretical foundation of the Sunyaev–Zel’dovich effect was laid in the early 1970s (Sunyaev and Zel’dovich, 1970), but is based on earlier work on the interactions of photons and free electrons (Kompaneets, 1956; Dreicer, 1964; Weymann, 1965). Excellent recent reviews of the physics of the Sunyaev–Zel’dovich effect have been given by Bernstein and Dodelson (1990) and Rephaeli (1995b), while discussions of the more general problem of comptonization of a radiation field by passage through an

The thermal Sunyaev–Zel’dovich effect

The results in Section 3indicate that passage of radiation through an electron population with significant energy content will produce a distortion of the radiation’s spectrum. In the present section the question of the effect of thermal electrons on the CMBR is addressed in terms of the three likely sites for such a distortion to occur:

  • 1.

    the atmospheres of clusters of galaxies,

  • 2.

    the ionized content of the Universe as a whole, and

  • 3.

    ionized gas close to us.

The non-thermal Sunyaev–Zel’dovich effect

As was noted in Section 3.3, a non-thermal population of electrons must also scatter microwave background photons, and it might be expected that a sufficiently dense relativistic electron cloud would also produce a Sunyaev–Zel’dovich effect. Fig. 11, which shows a radio map of Abell 2163 superimposed on a soft X-ray image, indicates that in some clusters there are populations of highly relativistic electrons (in cluster radio halo sources) that have similar angular distributions to the

The kinematic Sunyaev–Zel’dovich effect

Although early work on the Sunyaev–Zel’dovich effects concentrated on the thermal effect, a second effect must also occur when the thermal (or non-thermal) Sunyaev–Zel’dovich effect is present. This is the velocity (or kinematic) Sunyaev–Zel’dovich effect, which arises if the scattering medium causing the thermal (or non-thermal) Sunyaev–Zel’dovich effect is moving relative to the Hubble flow. In the reference frame of the scattering gas the microwave background radiation appears anisotropic,

Polarization and the Sunyaev–Zel’dovich effect

In the previous sections of this review I have concentrated on the Sunyaev–Zel’dovich effects in the specific intensity, the Stokes I parameter. Any effects in the polarized intensity, the Stokes Q,U, and V terms, will be of smaller order by factors τe, or v/c. An early reference was made to polarization terms in the paper by Sunyaev and Zel’dovich (1980b), with particular reference to their use to measure the velocities of clusters of galaxies across the line of sight. A more thorough

Measurement techniques

Three distinct techniques for the measurement of the Sunyaev–Zel’dovich intensity effects in clusters of galaxies are now yielding reliable results. This section reviews single-dish radiometric observations, bolometric observations, and interferometric observations of the effects, emphasizing the weaknesses and strengths of each technique and the types of systematic error from which they suffer. A discussion of the constraints on observation of the non-thermal effect is contained in the

Sunyaev–Zel’dovich effect data

The techniques discussed in Section 8have been used to search for the thermal and kinematic Sunyaev–Zel’dovich effects towards a large number of clusters, and the non-thermal Sunyaev–Zel’dovich effects towards a few radio galaxies. Over the past few years this work has been increasingly successful, because of the high sensitivity that is now being achieved, and the careful controls on systematic errors that are used by all groups. The most impressive results are those obtained from radio

The Sunyaev–Zel’dovich effect analysed in terms of cluster properties

The Sunyaev–Zel’dovich effects provide a window on cluster properties which differs significantly from that afforded by optical, X-ray, or conventional radio data. The present section of this review concentrates on these implications of the measurement of the effects for the understanding of cluster properties.

The Sunyaev–Zel’dovich effect interpreted in cosmological terms

The simplest cosmological use of the Sunyaev–Zel’dovich effect is to prove that the CMBR is genuinely a cosmological phenomenon: the appearance of an effect from a cluster of galaxies at z=0.5455 (CL 0016+16) proves that the CMBR originates at z>0.54, higher-redshift detections push this limit even further. However, it is as a probe of cosmological parameters, and as a distance-independent probe of earlier phases of the Universe that the Sunyaev–Zel’dovich effect has attracted most interest,

Continuing research and the future of the Sunyaev–Zel’dovich effect

Developments in the technologies of microwave background observation are continuing, so that there is every reason to expect that all clusters of galaxies with luminous X-ray emitting atmospheres will eventually be detected in their Sunyaev–Zel’dovich effects. Cm-wave measurements, with traditional single-dish telescopes and radiometers, are unlikely to be as effective, in the long run, as mm-wave measurements using bolometers simply because many strong X-ray clusters also contain bright radio

Acknowledgements

This review was partially supported by NASA grants NAGW-3825 and NAG5-2415, NASA contract NAS8-39073, and a research grant from PPARC. My research on the Sunyaev–Zel’dovich effects over the years has benefited from many collaborators, especially S.F. Gull, J.P. Hughes, H. Liang, A.T. Moffet, and S.M. Molnar, and the generous assistance of observatory staff at the Owens Valley Radio Observatory and the Very Large Array. I am also grateful to J.E. Carlstrom, M. Jones, M. Joy, J.-M. Lamarre, A.E.

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    Also Smithsonian Institution Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138, USA. E-mail: mark. [email protected].

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