eaa.iop.orgDOI: 10.1888/0333750888/2605 Superclusters and the Local SuperclusterR Brent Tully FromEncyclopedia of Astronomy & AstrophysicsP. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888Downloaded on Tue Feb 07 18:29:13 GMT 2006 [131.215.103.76]Institute of Physics PublishingBristol and PhiladelphiaTerms and ConditionsSuperclusters and the Local SuperclusterENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSSuperclusters and the LocalSuperclusterIn the 19th century, William and John Herschel noted theuneven distribution of spiral nebulae across the sky (seeHERSCHEL FAMILY). In the early 1920s, J H Reynolds remarkedthat large non-galactic nebulae cluster along a band and10 yr laterHARLOW SHAPLEY, providing confirmation with amore systematic survey, called the flattened concentrationof brightest galaxies the ‘Local Supergalaxy’. G´erard deVaucouleurs, in the 1950s, showed that locally there is aflattened distribution of galaxies with theVIRGO CLUSTER atits core and ourMILKY WAY GALAXY toward the periphery.This structure has come to be called the ‘Local Supercluster’.After the mid-1970s, the effort to study the distribution ofgalaxies in the universe intensified and many structuressimilar to the Local Supercluster have now been found.Galaxies: the building blocks of large-scalestructureIt is currently believed that structure in the universeformed from the gravitational self-attraction of matter inoverdenseregions. Dense regions collapsed earlier in time.Probably, irregularities with high local density but lowtotal mass formed first, then coalesced with other matter toform larger entities. Galaxies were forming and growingrapidly during the first four to six billion years in the lifeof the universe. All the while, galaxies were being drawncloser to each other, so they were becoming substantiallyclumped. The timescales for this gravitational clusteringare strongly dependent on the overall properties of theuniverse. If the average mass density is in the vicinity ofthe critical density for a closed universe then galaxies arefalling together in large numbers today to form large-scalestructures. If the average mass density is much less thanthe critical density then the greatest development in thestructure formation happened earlier.In their gross properties, galaxies can be consideredto be of two types. Galaxies referred to asELLIPTICALor LENTICULAR GALAXIES overwhelmingly contain only oldstars. They have little cold interstellar gas so they lackthe resources to form many new stars. On the other hand,galaxies calledSPIRAL or IRREGULAR GALAXIES do contain coldinterstellar gas and they have a substantial component ofyoung stars. These galaxies can have a wide range ofSTELLAR POPULATIONS, from very young to very old.Galaxies of the two types are usually found in verydifferent environments. Ellipticals and lenticulars arefound in large numbers in clusters, the densest, mostcongested parts of the universe. Spiral and irregulargalaxies are found in filamentary structures outside thedensest clusters. There are relatively few spirals or gas-rich irregulars in clusters. It is possible that those fewspirals in clusters have fallen into the gravitational wellsof the clusters recently. There are relatively few ellipticalsor lenticulars in the low-density regions. Outside of denseclusters, most galaxies have a supply of gas for new stars.Our own Milky Way Galaxy is a relatively large spiralgalaxy. The solar system is located in the flattened disknear a spiral arm spur far out from the center.Groups: the next level of the hierarchyGalaxies like to be with other galaxies. Gravity pullsthem together. Galaxies formed out of the self-attractionof matter in overdense regions and galaxies, in turn, areembedded in larger-scale overdense regions that collapseto form groups. Seventy per cent of galaxies are identifiedto lie in groups that are gravitationally bound and another20% are found in looser associations that may not bebound. The vast majority of these groups appear to bedynamically young. Although the galaxies are usuallybound gravitationally to each other, they may not havehad time to make many orbits, or even one orbit, aroundeach other. When they do approach, near collisions thatcause structural disruption are not uncommon.The dynamically young groups are predominantlypopulated with gas-rich spiral and irregular galaxies.There may also be swarms of dwarf spheroidal systemsabout big galaxies, to judge from our Milky Waysituation. TheseDWARF SPHEROIDAL GALAXIES are so faintthat they are difficult to see at the distances of otherbig galaxies. A small fraction, roughly 5%, of groupsare predominantly populated by gas-poor ellipticals andlenticulars. These groups are dense, have higher motionsand are undoubtedly more dynamically evolved.TheLOCAL GROUP is an example of a typicalenvironment for galaxies. There are two giant spirals,Andromeda or M31 (the biggest) and the Milky Way, andtwo intermediate-size galaxies, Triangulum or M33 andthe Large Magellanic Cloud. In total there are 40 knownor suspected members but most of these are dwarfs. Thereare two main subgroups within the Local Group, one eacharound the two giant galaxies. There are two other minorsubgroups around the irregular galaxies NGC 6822 andNGC 3109. The two giant galaxies contain 80% of the lightof the group and are falling toward each other.Clusters: the largest collapsed structures in theuniverseRich CLUSTERS OF GALAXIES act as nodal points in thefilamentary structure of the universe. Clusters can containhundreds of substantial galaxies and thousands of dwarfs.The total mass associated with big clusters can reach 1015times the mass of the Sun. Less than 10% of this mass isdirectly associated with the stars in the galaxies. Of order20% can be associated with hot hydrogen and helium gasthat fills the space between the galaxies in the clusters andradiates thermal x-ray emission. Most of the mass is in anunknown form and is part of the ‘DARK MATTER’ problem.The galaxies in the richest clusters are ellipticals,lenticulars and dwarf spheroidals for the most part.Spirals and irregulars fall into the clusters and over timemust exhaust or be stripped of their cold gas reservoirs.Down at the center of the cluster one frequently finds aspecial elliptical galaxy. The central ellipticals are oftenCopyright © Nature Publishing Group 2001Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998and Institute of Physics