[Previous] | [Session 111] | [Next]
M. E. Donahue (STScI)
The epoch from z=2.5-1.0 may well be the epoch of cluster formation. Today, studying this epoch from the perspective of clusters is nearly impossible. We study clusters to investigate cosmological parameters, the assembly of large scale structure, the chemical enrichment history of the universe, and the effects of environment and birth place on the evolution of galaxies. But clusters are rare objects; cluster-finding requires deep imaging of large areas of sky (the most massive clusters have a space density of only 10-9 h503 Mpc-3). Clusters, because of the effects of cosmological diminuition on their X-ray surface brightness at X-ray wavelengths, and the K-correction effects of their member galaxies (predominately ellipticals in the cores), are very difficult to detect in the X-rays or at optical wavelengths at z>1-1.5. A wide-field survey with the sensitivity of HAB~27 would enable us to detect incipient elliptical galaxies 2 mags below present-day L* out to z~2-2.5. A 1,000 deg2 cluster survey including ~2000 of the most massive clusters would open cluster studies to previously unaccessible territory: the epoch from z=1-2.5. Clusters have not evolved very much over the epoch we can now study them, z=0-1. However, from theoretical expectations regarding the formation of large scale structure, we expect clusters to be evolving very rapidly in the epoch z=2.5-1.0, the era of cluster formation. Finding and studying clusters in this redshift range would open an important window on cluster and structure evolution.