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).