Annu. Rev. Astron. Astrophys. 2002. 40: 539-577 Copyright © 2002 by Annual Reviews. All rights reserved

3.4. Other methods

X-ray and optical surveys have been by far the most exploited techniques for studying the distribution and evolution of galaxy clusters. It is beyond the scope of this paper to review other cluster-finding methods, which we only summarize here for completeness:

• Search for galaxy overdensities around high-z radio galaxies or AGN: searches are conducted in near-IR or narrow-band filters, or by means of follow-up X-ray observations. Although not suited for assessing cluster abundances, this method has provided the only examples of possibly virialized systems at z > 1.5 (e.g. Pascarelle et al. 1996; Dickinson 1997; Crawford & Fabian 1996, Hall & Green 1998; Pentericci et al. 2000; Fabian et al. 2001b, Venemans et al. 2002).

• Sunyaev-Zeldovich effect: clusters are revealed by measuring the distortion of the CMB spectrum owing to the hot ICM. This method does not depend on redshift and provides reliable estimate of cluster masses. It is possibly one of the most powerful methods to find distant clusters in the years to come. At present, serendipitous surveys with interferometric techniques (e.g. Carlstrom et al. 2001) cannot cover large areas (i.e. more than ~ 1 deg²) and their sensitivity is limited to the most X-ray luminous clusters.

• Gravitational lensing: in principle a powerful method to discover mass concentrations in the universe through the statistical distortion of background galaxy images (see Mellier 1999 for a review).

• Search for clusters around bent-double radio sources: radio galaxies with bent lobes are often associated with dense ICM and are therefore good tracers of rich cluster environments (e.g. Blanton et al. 2001).

• Clustering of absorption line systems: this method has lead to a few detections of "proto-clusters" at z 2 (e.g. Francis et al. 1996). The most serious limitation of this technique is the small sample volume.