eaa.iop.orgDOI: 10.1888/0333750888/1668 Galaxy ClustersRaymond Carlberg FromEncyclopedia of Astronomy & AstrophysicsP. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888Downloaded on Tue Feb 07 18:37:49 GMT 2006 [131.215.103.76]Institute of Physics PublishingBristol and PhiladelphiaTerms and ConditionsGalaxy ClustersENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSGalaxy ClustersGalaxy clusters are the largest stable structures in theuniverse. Galaxy clusters have been key astrophysicalobjects in the development of our current understandingof a host of issues.• Dark matter was first detected in galaxy clusters.• Galaxy clusters contain gas at a temperature of about108K and are very luminous x-ray sources.• The theory of gravitational instability, which is thecurrent paradigm for the formation of all structure, wasfirst developed to understand galaxy clusters.• Clusters contain a unique mixture of galaxy types,including the biggest and many of the oldest.Galaxy clusters werefirst tentatively recognizedin thesky surveys ofNEBULAE in the 19th century. The two mostprominent in the northern sky are theVIRGO CLUSTER, whichis the approximate center of the local supercluster at adistance of about 20 Mpc from the Milky Way, and theCOMACLUSTER(figure 1), which is a spectacularly ‘rich’ cluster atREDSHIFT of about 6900 km s−1. The Virgo cluster lies nearthe north Galactic pole and provides us with a convenientlocal sample of a relatively low-richness cluster. Coma isalso located in the north Galactic cap and stands out as anunusual structure in the whole northern extragalactic sky.When galaxy clusters were first noticed in the 1800s,the size and structure of our own Galaxy was notknown, let alone the distinction between various typesof nebulae in our Galaxy and the extragalactic nebulae,which turned out to be other galaxies. Hubble’s distancescale of the late 1920s was an essential underpinning tothe physical investigation of galaxy clusters. The firststep was momentous.ZWICKY measured the velocities ofindividual galaxies in the Coma cluster. Then, making thesomewhat bold assumption that the cluster was in a stableequilibrium, he derived its total mass. The ratio of the massto the total luminosity of cluster galaxies is an astonishingvalue of about 200M per L (adjusted to current-dayvalues of theHUBBLE CONSTANT). The Sun itself has a mass-to-light ratio of unity, and mixes of stars in an averagegalaxy have M/L values ranging from about 1 to about10, depending on their Hubble type. Zwicky’s conclusionwas that most of the mass was ‘missing’ from the censusof visible light. This conclusion was controversial untilthe 1970s, the primary issue being the dynamical stateof clusters which was unclear partly because there wasno clear theory of how such massive objects could cometogether. In the late 1960s Peebles (1970) did one of thefirst n-body simulations to show how a cluster could formfrom small density fluctuations in an expanding universe.Then, with the first imaging x-ray telescopes in space itwas conclusively established that large clusters of galaxiescontained gas at a temperature of about 108K. Such hot gasrequired that there be a large, deep potential well to keep itcontained. X-ray observations of clusters have become oneof the major tools for first finding galaxy clusters and forinvestigating the structure of theDARK MATTER distribution.In the 1980sGRAVITATIONAL LENSING was discovered ingalaxy clusters, more or less as predicted by Zwicky.This elegant technique is both simple and powerful.The data consist of nothing other than a picture of acluster at a moderate redshift, say roughly 1/2, wherethe lensing effect has a reasonable ‘lever arm’; the picturewill have a high density of background galaxies withouttoo many foreground sources (which suffer no lensingeffect). The gravitational field of the cluster causes thebackground galaxies to be slightly tangentially elongated(on the average), which directly measures the meansurface density of the cluster. Very deep images withexceptionally high angular resolution (such as from theHubble Space Telescope) reveal more detail which canbe used to investigate smaller-scale structure in the darkmatter distribution.Afamous theoretical paper of Sunyaev and Zeldovich(SZ) predicted that asCOSMIC MICROWAVE BACKGROUND (CMB)radiation passes through a cluster its radiation would beaffected in two ways. First, the very hot electrons in thecluster would Compton scatter the 3 K photons of theCMB, leading to a slight increase in the energy of eachphoton. If the cluster is moving with respect to the CMB,then there is an additional Doppler effect of light beingred- or blueshifted by reflection from a moving mirror.By comparing the CMB radiation near to the cluster withthat seen coming through the cluster theSUNYAEV–ZELDOVICHEFFECTcan be used to map the distribution of hot gasin clusters. This technique is best done at fairly shortwavelengths where the confusing effects of radio sourcesare minimized. Radio telescopes that operate efficientlyfor these measurements at very high frequencies are nowbeing constructed.The galaxy content of clusters is unique. TheGALAXYMORPHOLOGY–DENSITY RELATION(Dressler) states that the low-density field is dominated by spiral-type galaxies but asone moves to regions of ever higher density the fraction ofspirals declines and elliptical and S0 galaxies dominate inregions of very high galaxy density, such as the centersof galaxy clusters. Spirals are actively forming starswhereas E and S0 galaxies have very low star formationrates. Understanding this relation is a key consideration intheories ofGALAXY FORMATION. Although E and S0 galaxiesare fairly rare outside of clusters, they do provide about15% or so of the field galaxy mix. The cD galaxytype, a supergiant elliptical with an extended envelope,is unique to clusters. Relative to field galaxies, clustergalaxies are older and contain a larger fraction of theirstars in a spheroidal component as opposed to a flat disk.One possible inference is that galaxy merging was moreimportant in clusters than in the field.Finding galaxy clustersClusters are rare objects, containing about 1% of allgalaxies. Finding them efficiently with minimal biasesremains a research problem. The problem is that theCopyright © Nature Publishing Group 2001Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998and Institute of Physics Publishing 2001Dirac House,