Optical Detection of Clusters of Galaxies

Abstract

Taken literally, galaxy clusters must be comprised of an overdensity of galaxies. Almost as soon as the debate was settled on whether or not the “nebulae” were extragalactic systems, it became clear that their distribution was not random, with regions of very high overand under-densities. Thus, from a historical perspective, it is important to discuss the detection of galaxy clusters through their galactic components. Today we recognize that galaxies constitute a very small fraction of the total mass of a cluster, but they are nevertheless some of the clearest signposts for detection of these massive systems. Furthermore, the extensive evidence for differential evolution between galaxies in clusters and the field means that it is imperative to quantify the galactic content of clusters. Perhaps even more importantly, optical detection of galaxy clusters is now inexpensive both financially and observationally. Large arrays of CCD detectors on moderate sized telescopes can be utilized to perform all-sky surveys with which we can detect clusters to z ∼ 0.5. Using some of the efficient techniques discussed later in this section, we can now survey hundreds of square degrees for rich clusters at redshifts of order unity with 4-m class telescopes, and similar surveys, over smaller areas but with larger telescopes are finding group-mass systems to similar distances. Looking to the future, ever larger and deeper surveys will permit the characterization of the cluster population to lower masses and higher redshifts. Projects such as the Large Synoptic Survey Telescope (LSST) will map thousands of square degrees to very faint limits (29th magnitude per square arcsecond) in at least five filters, allowing the detection of clusters through their weak lensing signal (i.e., mass) as well as the visible galaxies. Ever more efficient cluster-finding algorithms are also being developed, in an effort to produce catalogs with low contamination by line-of-sight projections, high completeness, and well-understood selection functions.