Searches for High Redshift Clusters

High redshift galaxy clusters have traditionally been a fruitful place to study galaxy evolution. I review various search strategies for finding clusters at z > 1. Most efforts to date have concentrated on the environments of distant AGN. I illustrate these with data on the cluster around 3C 324 (z = 1.2) and other, more distant systems, and discuss possibilities for future surveys with large telescopes. 1 Finding distant clusters At one time, galaxy clusters served as the most observationally straightforward means of studying galaxy evolution at high redshift. The reason was primarily one of contrast: even without spectroscopy, very rich clusters are recognizable as enhancements in the galaxy surface density out to z ∼ 1, and the properties of the cluster galaxy population can be studied statistically with imaging data alone if a proper “control sample” of field galaxies can be observed in the same manner. In this way, Butcher and Oemler (1978, 1984) provided the first convincing evidence for galaxy evolution, identifying an apparently systematic bluing trend for galaxies in the cores of rich clusters beyond z ≈ 0.3. At very large redshifts (z > 1), however, even quite rich clusters are no longer so clearly visible against the tremendously numerous population of faint field galaxies, and thus become correspondingly more difficult to discover and study. This is illustrated by figure 1, a cartoon representing the peak surface density contrast in the I–band of identical rich clusters observed at z = 0.3 and z = 1.2. Because the angular scale changes only slowly with redshift beyond z = 0.3, there is very little boost in the galaxy surface density as one moves a cluster further away, while the combination of distance modulus and k–correction dims the cluster galaxies considerably. The result is that a cluster with a contrast 10× over the field galaxy population at z = 0.3 barely peaks up over the field at all at z = 1.2. The effects of the k–correction are much more severe for early–type galaxies, rendering a rich, elliptical dominated cluster nearly invisible at optical wavelengths for z > 1, even assuming reasonable amounts of passive evolution. On the one hand, thanks to highly efficient multiplexing spectrographs, the tremendous progress which has occurred in the study of high redshift field galaxies has meant that clusters are no longer “needed” to provide large samples of distant galaxies. But at the same time, the higher the redshift, the more interesting a rich cluster becomes from a cosmological viewpoint. Firstly, as the most massive collapsed structures in the universe, their properties and evolution are highly sensitive to the fundamental cosmological parameters, as well as to the power spectrum of mass fluctuations which give rise to large scale structure in