Historically, identifying galaxy clusters from overdensities on deep photographic plates was a proven method to find distant galaxies. Because the central cD galaxy in a cluster is quite luminous relative to the average galaxy [MV(cD) -24 as opposed to MV* -21.5 for early-type galaxies in clusters], clusters are excellent high-luminosity landmarks for studying the extragalactic universe and were used by astronomers from 1950 to 1980 to find galaxies out to z 0.6. While no longer primary sites for identifying distant galaxies directly because clusters require a substantial fraction of the Hubble time to form and virialize, they are once again becoming important for studying the process of structure formation. Eke, Cole, & Frenk (1996) show that the evolution of cluster abundances is sensitive to basic cosmological parameters. The temporal evolution of the comoving galaxy cluster number density is determined by the rate of growth of large density perturbations. This depends mostly on the value of ; in a low-density universe the cluster population evolves slowly at low to intermediate redshift. In a critical ( = 1) universe the density fluctuations continue to grow to the present epoch - thus, the cluster population is still evolving, and we should expect more clusters locally than at intermediate redshift (z 0.8). Preliminary results from deep X-ray surveys do not indicate significant evolution in the galaxy cluster population from z 0.8 to the local universe, suggestive of a low- universe (e.g., Rosati 2000).
The gravitational lensing caused by massive galaxy clusters amplifies the apparent brightness of background sources. Several galaxies at z > 4 have now been identified behind Abell clusters (e.g., Frye & Broadhurst 1998), with the most distant strongly lensed source a serendipitously discovered system at z = 4.92 (Franx et al. 1997).