eaa.iop.org DOI: 10.1888/0333750888/2605 Superclusters and the Local Supercluster R Brent Tully From Encyclopedia of Astronomy & Astrophysics P. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888 Institute of Physics Publishing Bristol and Philadelphia Downloaded on Tue Feb 07 18:29:13 GMT 2006 [131.215.103.76] Terms and Conditions Superclusters and the Local Supercluster E NCYCLOPEDIA OF A STRONOMY AND A STROPHYSICS Superclusters and the Local Our own Milky Way Galaxy is a relatively large spiral galaxy. The solar system is located in the flattened disk Supercluster near a spiral arm spur far out from the center. In the 19th century, William and John Herschel noted the Groups: the next level of the hierarchy uneven distribution of spiral nebulae across the sky (see Galaxies like to be with other galaxies. Gravity pulls HERSCHEL FAMILY). In the early 1920s, J H Reynolds remarked them together. Galaxies formed out of the self-attraction that large non-galactic nebulae cluster along a band and of matter in overdense regions and galaxies, in turn, are 10 yr later HARLOW SHAPLEY, providing confirmation with a embedded in larger-scale overdense regions that collapse more systematic survey, called the flattened concentration to form groups. Seventy per cent of galaxies are identified of brightest galaxies the ‘Local Supergalaxy’. G´erard de to lie in groups that are gravitationally bound and another Vaucouleurs, in the 1950s, showed that locally there is a 20% are found in looser associations that may not be flattened distribution of galaxies with the VIRGO CLUSTER at bound. The vast majority of these groups appear to be its core and our MILKY WAY GALAXY toward the periphery. dynamically young. Although the galaxies are usually This structure has come to be called the ‘Local Supercluster’. bound gravitationally to each other, they may not have After the mid-1970s, the effort to study the distribution of had time to make many orbits, or even one orbit, around galaxies in the universe intensified and many structures each other. When they do approach, near collisions that similar to the Local Supercluster have now been found. cause structural disruption are not uncommon. The dynamically young groups are predominantly Galaxies: the building blocks of large-scale populated with gas-rich spiral and irregular galaxies. structure There may also be swarms of dwarf spheroidal systems It is currently believed that structure in the universe about big galaxies, to judge from our Milky Way formed from the gravitational self-attraction of matter in situation. These DWARF SPHEROIDAL GALAXIES are so faint overdense regions. Dense regions collapsed earlier in time. that they are difficult to see at the distances of other Probably, irregularities with high local density but low big galaxies. A small fraction, roughly 5%, of groups total mass formed first, then coalesced with other matter to are predominantly populated by gas-poor ellipticals and form larger entities. Galaxies were forming and growing lenticulars. These groups are dense, have higher motions rapidly during the first four to six billion years in the life and are undoubtedly more dynamically evolved. of the universe. All the while, galaxies were being drawn The LOCAL GROUP is an example of a typical closer to each other, so they were becoming substantially environment for galaxies. There are two giant spirals, clumped. The timescales for this gravitational clustering Andromeda or M31 (the biggest) and the Milky Way, and are strongly dependent on the overall properties of the two intermediate-size galaxies, Triangulum or M33 and universe. If the average mass density is in the vicinity of the Large Magellanic Cloud. In total there are 40 known the critical density for a closed universe then galaxies are or suspected members but most of these are dwarfs. There falling together in large numbers today to form large-scale are two main subgroups within the Local Group, one each structures. If the average mass density is much less than around the two giant galaxies. There are two other minor the critical density then the greatest development in the subgroups around the irregular galaxies NGC 6822 and structure formation happened earlier. NGC 3109. The two giant galaxies contain 80% of the light In their gross properties, galaxies can be considered of the group and are falling toward each other. to be of two types. Galaxies referred to as ELLIPTICAL or LENTICULAR GALAXIES overwhelmingly contain only old Clusters: the largest collapsed structures in the stars. They have little cold interstellar gas so they lack universe the resources to form many new stars. On the other hand, Rich CLUSTERS OF GALAXIES act as nodal points in the galaxies called SPIRAL or IRREGULAR GALAXIES do contain cold filamentary structure of the universe. Clusters can contain interstellar gas and they have a substantial component of hundreds of substantial galaxies and thousands of dwarfs. young stars. These galaxies can have a wide range of The total mass associated with big clusters can reach 1015 STELLAR POPULATIONS, from very young to very old. times the mass of the Sun. Less than 10% of this mass is Galaxies of the two types are usually found in very directly associated with the stars in the galaxies. Of order different environments. Ellipticals and lenticulars are 20% can be associated with hot hydrogen and helium gas found in large numbers in clusters, the densest, most that fills the space between the galaxies in the clusters and congested parts of the universe. Spiral and irregular radiates thermal x-ray emission. Most of the mass is in an galaxies are found in filamentary structures outside the unknown form and is part of the ‘DARK MATTER’ problem. densest clusters. There are relatively few spirals or gas- The galaxies in the richest clusters are ellipticals, rich irregulars in clusters. It is possible that those few lenticulars and dwarf spheroidals for the most part. spirals in clusters have fallen into the gravitational wells Spirals and irregulars fall into the clusters and over time of the clusters recently. There are relatively few ellipticals must exhaust or be stripped of their cold gas reservoirs. or lenticulars in the low-density regions. Outside of dense Down at the center of the cluster one frequently finds a clusters, most galaxies have a supply of gas for new stars. special elliptical galaxy. The central ellipticals are often Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK 1 Superclusters and the Local Supercluster E NCYCLOPEDIA OF A STRONOMY AND A STROPHYSICS Coma Cluster Great Wall Southern Wall Figure 1. Large-scale structure (Luiz da Costa (European Southern Observatory), Margaret Geller (Harvard-Smithsonian), and John Huchra (Harvard-Smithsonian). particularly large and puffed up. They probably grow Filaments of galaxies: the cosmic web through the ACCRETION of galaxies through collisions and Ninety per cent of galaxies lie outside of rich clusters in dynamical friction. The hot intracluster gas can slowly extended filaments or ‘clouds’. Individual filaments are condense onto the central galaxy through a cooling flow. long in one dimension, often intermediate in a second dimension, and thin in the third dimension, like a flat pasta By far the most prominent nearby cluster is the or a surfboard. Adjacent filaments can share a common Virgo cluster, usually taken as the center of the Local plane to give the impression of a partially filled surface. Supercluster. The Virgo cluster is at a distance of 16 Mpc Other adjacent filaments can lie in layers, analogous to (50 million light-years). Galaxies in a spherical region window louvers. The groups of galaxies lie like beads within the within 7 Mpc have decoupled from the expansion of the filaments. It is thought that filaments have grown through universe and are falling back onto the cluster. self-attraction except that collapse is only partial. Gravity The nearest rich clusters beyond the Local Super- has not overcome the expansion of the universe in at least ∼ cluster are HYDRA I and CENTAURUS clusters about 2.5 times the long dimension. In addition to the 90% of galaxies in more distant than the Virgo cluster, the Perseus cluster bound groups or looser associations, about 10% of galaxies are not closely linked with other galaxies but still lie in about 4 times more distant and the particularly rich the filaments. At most only a few per cent of galaxies lie Coma cluster about 5 times more distant. There are outside these structures (see the complementary article on occasional aggregates of rich clusters, such as the ‘SHAPLEY VOIDS). CONCENTRATION’ of 29 clusters at a distance of roughly Although not conclusively demonstrated, it is 200 Mpc. strongly indicated that filaments essentially never stand Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK 2 Superclusters and the Local Supercluster E NCYCLOPEDIA OF A STRONOMY AND A STROPHYSICS Figure 2. Detail of part of figure 1 (R Brent Tully (University of Hawaii)). alone. Any two places in the universe that are on filaments The most certain evidence for motion comes from may be interconnected by a path that runs through the the dipole distribution in the COSMIC MICROWAVE BACKGROUND network of filaments. Simulations of the formation temperature maps. It is concluded that the Milky Way − of structure demonstrate that matter collapses onto the Galaxy has a motion of 600 km s 1 toward a well- filaments in one or two dimensions and then drains along defined position in the southern celestial sky.