eaa.iop.orgDOI: 10.1888/0333750888/1822 Virgo ClusterBruno Binggeli, John Huchra FromEncyclopedia of Astronomy & AstrophysicsP. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888Downloaded on Thu Mar 02 23:40:15 GMT 2006 [131.215.103.76]Institute of Physics PublishingBristol and PhiladelphiaTerms and ConditionsVirgo ClusterENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSVirgo ClusterThe Virgo cluster is the nearest and best-studied richCLUSTER OF GALAXIES, lying at a distance of about 55 millionlight-years in the constellation ofVIRGO. Figure 1 is amap of the distribution of nearby galaxies (as determinedby theirREDSHIFTS) in a 1500 square degree region ofthe constellation of Virgo. The Virgo cluster is thestrong, somewhat irregular, concentration of galaxies atthe center. In three-dimensional (3D) space, the Virgocluster constitutes the nucleus of the Local Supercluster(LSC) of galaxies, in whose outskirts ourMILKY WAY GALAXYis situated.As early as 1784,CHARLES MESSIER noted an unusualgroup of ‘nebulae’ in Virgo. Fifteen out of the 109 famous‘Messier objects’ are, in fact, member galaxies of the Virgocluster. However, only in the 1920s, followingEDWINHUBBLE’s proof of theextragalactic nature of those ‘nebulae’,was Messier’s group understood as a self-gravitatingsystem of hundreds of galaxies, and the first systematicinvestigations of the Virgo cluster, as it was henceforthcalled, were carried out byHARLOW SHAPLEY and others.Virgo was the first galaxy cluster to be studieddynamically by Smith and Zwicky in the 1930s. Theirwork showed that the dynamical mass of Virgo, estimatedby using the virial theorem2T + U = 0where T and U are the time-averaged kinetic energyand potential energy of the relaxed system, was muchlarger than the mass inferred by integrating the lightof all the galaxies in the cluster and multiplying by amass-to-light ratio (M/L) like the average of stars in thesolar neighborhood. This was the first clear detectionof ‘DARK MATTER,’ or more properly, non-luminous mass.The distribution of galaxies in the direction of Virgo inredshift space is shown in figure 2, and figure 3 displays thehistogram of galaxy velocities inside a circle of 6◦radiuscentered on the core of the cluster at 12h28mand +10◦28 .The Virgo cluster lies at the center of the LSC,first dynamically studied by G de Vaucouleurs andcollaborators in the 1950s. The Milky Way appears tobe falling into the Virgo cluster, relative to the generalexpansion of the universe, with a velocity of ∼250 km s−1.That is to say, we are still moving away from the clustercore with an apparent velocity, cz,of∼1100 km s−1, butthat velocity is 250 km s−1less than it would have beenif the cluster had no mass. Curiously, the M/L for thewhole LSC derived from this Virgo infall is similar to thatderived via the virial theorem for the dynamically relaxedcluster core; both M/L values, when applied to the meanluminosity density of the universe derived from redshiftsurveys, imply a mean matter density of the universe thatis only 1/4 to 1/3 of the critical density.The kinematics of the cluster is quite complex ascan be seen in figures 1 and 4. There are two mainconcentrations of galaxies; the largest and densest iscentered on the well-known galaxyM87 (NGC4486) whichis both a strong radio source (VIRGO A) and a strong x-ray source. This subcluster, at 12h30m.8+12◦23 , is also astrong x-ray source produced by a reservoir of extremelyhot intracluster gas. ThisINTRACLUSTER MEDIUM is at atemperature of ∼107K, emits thermal bremsstrahlungradiation and is a major contributor to the total mass ofthe system. The southern large subcluster is centered onM49 (NGC 4472) at 12h29m.8+8◦00 , the most luminousgalaxy in the cluster, but is significantly less massive thanthe M87 subcluster. The total mass of the Virgo core regionis ∼1015M, that of the M87 subclump ∼3 × 1014Mandthe M49 subclump is ∼1 × 1014M, assuming a distanceto the cluster core of 16 Mpc. The cluster distance of∼16 Mpc has been estimated via a variety of techniquesincluding measurements of Cepheids to six galaxies withthe Hubble Space Telescope, studies of globular clustersand planetary nebulae (PNe) luminosity functions, andsurface brightness fluctuations (SBFs).At present, approximately 1300 member galaxies ofthe Virgo cluster are known, most of which are very faintDWARF GALAXIES (but this number is bound to grow fastin the near future, as ever fainter members are going tobe detected with improving techniques). Their projectedpositions within the cluster—spread over a large sky areaof roughly 100 square degrees, or 500 full moons (seefigure 4)—reveal a rich substructure. The Virgo clusterindeed represents the most common type of clusters ofgalaxies that are loosely concentrated and ‘irregular’.Several gravitationally bound subsystems of galaxies havebeen identified. Of these, the subcluster centered onMessier 87 is by far the largest and most massive structure(the Virgo cluster proper, so to speak)—and M87 itself,the giant, active galaxy with its famous jet, can truly beregarded as the heart of the Virgo cluster. The smallersubclusters seem to be in a state of merging with the M87subcluster. In this sense, the Virgo cluster (like many, ifnot most, clusters of galaxies) is still in the making.The Virgo cluster has always been, and still is, one ofthe most important stepping stones for the cosmologicaldistance scale. Much of the current debate on the value oftheHUBBLE CONSTANT boils down to a debate on the meandistance of the Virgo cluster (see below).A new and very exciting era of Virgo cluster researchhas recently been opened by the first detection ofintraclusterPLANETARY NEBULAE (PNe) and single RED GIANTSTARS, i.e. stellar objects that are not bound to a single galaxybut to the cluster as a whole. This adds a whole newpopulation to the cluster, which in the future will be usedto explore the 3D structure and dynamical history of theVirgo cluster.Global structureThe primary optical database for the galaxy content ofthe Virgo cluster is the Las Campanas photographicsurvey, carried out in the 1980s by Allan Sandage andcollaborators, and encapsulated in the Virgo ClusterCatalog (VCC). Figure 4 is a map of the ca 1300 galaxies inCopyright © Nature Publishing Group 2001Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998and Institute of Physics Publishing 2001Dirac House, Temple Back,