eaa.iop.orgDOI: 10.1888/0333750888/2600 Coma ClusterMatthew Colless FromEncyclopedia of Astronomy & AstrophysicsP. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888Downloaded on Thu Mar 02 22:50:05 GMT 2006 [131.215.103.76]Institute of Physics PublishingBristol and PhiladelphiaTerms and ConditionsComa ClusterENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSComa ClusterComa, with Virgo, is one of the best-studied GALAXYCLUSTERS. The cluster is located at right ascension12h59m48.7s, declination +27◦5850(J2000) and lies almostat the north Galactic pole, (l, b) = (58◦, +88◦). In themajor catalog of galaxy clusters by Abell, its designationis Abell 1656 (seeABELL CLUSTERS). The mean redshift of thecluster is approximately 6900 km s−1, which puts it at adistance of 69h−1Mpc (where H0= 100h km s−1Mpc−1).Coma is one of the richest nearby clusters, having650 confirmed member galaxies and, including dwarfs,probably as many as 2000 galaxies in total. It has twocentral, dominant galaxies and therefore is classified as abinary cluster.Coma has long been taken as the archetypal richcluster. It appears regular and roughly spherical, witha strong central concentration. The spherical symmetryand general regularity meant that it was considered to bea good (if rare) example of a cluster that had achieveddynamical equilibrium and was therefore amenable tostraightforward theoretical analysis. Work over the lasttwo decades, however, has gradually revealed that Comais far from relaxed, with many substructures. Thecomplexity of the dynamical system reveals clues to theprocess of cluster formation.The total mass of Coma has been estimated fromobservations of its gravitational effects on the galaxies andhot x-ray gas in the cluster. Assuming that the cluster is indynamical equilibrium, the total mass is found to be muchgreater than the observed mass in the galaxies and x-raygas. This result, first obtained byZWICKY in 1933, remainsone of the strongest pieces of evidence that the universeis predominantly composed of some form of unseenDARKMATTER.HistoryWilliam Herschel was the first to note the concentrationof nebulae in the constellationCOMA BERENICES in 1785 (seefigure 1). The first catalog of 108 nebulae in Coma wasgiven by Wolf in 1902 and expanded by Curtis to over 300objects in 1918. The recession velocities for a few membersof the cluster were measured by Hubble and Humason in1931.The place of the Coma cluster as a touchstone ofextragalactic astronomy and cosmology was establishedwhen Zwicky derived an estimate for the mass of thecluster in 1933 and found it to be at least 2 × 1014h−1M,which was much greater than could be accounted for bythe visible matter. This was the first evidence suggestingthat some form of dark matter existed in the universe.Cluster galaxiesRichnessComa has 276 member galaxies brighter than B = 18within 0.75h−1Mpc (0.67◦) of the cluster center andapproximately 900 members brighter than B = 20 out to1.5h−1Mpc (1.33◦). The distance modulus for Coma is34.2−5 log h, so that B = 20 corresponds to MB=−14.2;about half the members of the Local Group of galaxies aredwarf galaxies with absolute magnitudes fainter than this,so the total number of galaxies in Coma is about 2000.Luminosity functionThe number of galaxies as a function of luminosity hasbeen determined in Coma at optical, infrared, ultravioletand radio wavelengths. The optical and near-infraredluminosity function (LF) is not very well fitted by thestandard Schechter functional form and appears to havethree main regimes: the number of galaxies per unityluminosity increases rapidly between the brightest clustermember at B = 12.6 and B ≈ 16 (L ≈ 3 × 109h−1L), andthen levels off to B ≈ 18 (L ≈ 0.5 × 109h−1L), beforeincreasing as a power law, N(L) ∝ Lα, for the dwarfgalaxies at fainter magnitudes. The LF varies with positionin the cluster. In the central regions the LF in fact has a peakat around B ≈ 16.5 and a dip at B ≈ 17.5 before risingsteeply. This may be due to mass (luminosity) segregationas the cluster approaches equipartition of energy in thecenter, or due to increased merging in the cluster coreor due to different evolutionary histories for the centralgalaxies. The steepness of the power law for dwarfgalaxies also depends on position in the cluster, going fromα ≈−1.3 in the core to α ≈−1.8 on the periphery.MorphologiesAs in most clusters, the galaxies in the core of Comaare mostly ellipticals and lenticulars, while the outskirtshave a higher proportion of spirals. For the brightest 200galaxies in the cluster, the mix is approximately E:S0:Sp =30%:55%:15%. The variation in the morphological mix isreflected in the colors of the galaxies, with red galaxiesdominating in the center and blue galaxies on theperiphery. There is an excess of early-type galaxies inthe region between the NGC4839 subcluster and the maincluster that show evidence of recent star formation, whichmay have been triggered by interaction with the tidalfield of the cluster or the intracluster medium (ICM) asthese objects fell into Coma. More generally, the E and S0galaxies in the cluster are strongly concentrated along anortheast–southwest axis while the spirals show a morediffuse and isotropic distribution.Dominant galaxiesThere are three dominant (D or cD) galaxies in the Comacluster. Two of these (NGC4874 and NGC4889) lie inthe cluster core and cause it to be classified as a binarycluster (Bautz–Morgan class II). NGC4874 (B = 12.8)is a cD galaxy with an extended halo and an absoluteluminosity of 5.8 × 1010h−1Lthat is located close tothe peak of the distributions of galaxies and x-ray gas.NGC4889, 0.12◦east of NGC4874, is even brighter (B =12.6, 6.9 × 1010h−1L), although it lacks an extended halo.Both NGC4874 and NGC4889 are estimated to have masses(within 80h−1kpc) of 1.4×1013h−1M. NGC4874 is a strongradio source but NGC4889 is not. The third dominantCopyright © Nature Publishing Group 2001Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998and Institute of Physics Publishing 2001Dirac House, Temple Back, Bristol, BS1 6BE, UK1Coma ClusterENCYCLOPEDIA OF ASTRONOMY AND A STROPHYSICSNGC 4921NGC 4911NGC 4841NGC 4839NGC 4889NGC 4874Figure 1. The visual appearance of the central region of the Coma cluster. This image, from the Palomar Observatory Sky Survey, iscentered at 12h59m15s+27◦5500(J2000) and covers 1.2◦×