eaa.iop.orgDOI: 10.1888/0333750888/1671 Groups of GalaxiesJoshua E Barnes FromEncyclopedia of Astronomy & AstrophysicsP. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888Downloaded on Tue Feb 07 18:37:09 GMT 2006 [131.215.103.76]Institute of Physics PublishingBristol and PhiladelphiaTerms and ConditionsGroups of GalaxiesENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSGroups of GalaxiesGravity creates a huge variety of cosmic structure, but mostgalaxies are found in groups of no more than a few dozenmembers. Groups range from the satellite systems of giantgalaxies to loose associations a few times denser than theirsurroundings to compact cores of rich galaxy clusters. Thecommon thread linking these examples is that the galaxiesmaking up a group interact more with each other than theydo with the rest of the universe; thus a group is a dynamicalunit.Groups are important for the evolution of galaxiesand of large-scale structures.GALAXY FORMATION appearsto be a drawn-out process, involving the collapse ofprimordial perturbations, accretion of gas and dark matter,outright merging of distinct objects and outflows of gasenriched by supernovae. Most galaxies conduct thesetransactions in group environments. However, groupenvironments are unstable; while the galaxies in a groupare forming, the group itself may be separating out fromthe cosmic expansion, collapsing under the influence ofgravity, accreting new members and finally merging withother groups to build clusters and superclusters.We can study the present outcome of these ongoingprocesses in our immediate vicinity and glimpse someearlier stages at highREDSHIFT. However, reconstructingthe evolution of groups is a difficult problem. Twopowerful tools which complement optical studies arecomputer simulations and multiwavelength observations.Simulations model the key dynamical ingredients ofgroups, including theDARK MATTER IN GALAXIES which isotherwise inaccessible. Observations at radio and x-ray wavelengths show how intergalactic gas responds asgroups evolve.Evolutionary stagesFigures 1 through 4 show four groups, ordered by stageof development. Group evolution begins when a boundconfiguration of several galaxies collapses out of theHubble flow; theLOCAL GROUP (LG) is now at this stage,with the Milky Way (MW) and M31 approaching eachother for the first time (figure 1). One of our nearestneighbors, the M81 group (figure 2), illustrates a moreadvanced stage; as the HI image shows, three galaxies arelinked by a complex structure indicating that they havealready undergone at least one passage.STEPHAN’S QUINTET(figure 3), a compact group, contains an extended, possiblytidal feature in neutral hydrogen and a central cloud ofhot gas visible in x-rays; this gas may have been heated bya fast, interpenetrating encounter between two galaxies.Finally, V Zw 311 (figure 4) is actually the core of clusterAbell 407; it is likely that several of these galaxies willmerge over the next ∼1 Gyr.These four examples represent not only differentevolutionary stages but also different evolutionary tracks.For example, the LG will become more compact as the MWand M31 draw closer, possibly evolving into a system likethe M81 group, but probably not attaining the dramaticstatus of Stephan’s Quintet, let alone V Zw 311.CollapseThe LG (figure 1) is probably the best place to study acollapsing group. Most of its luminosity is associatedwith the MW and M31, which are currently separatedby ∼0.73 Mpc and approaching each other at 120 km s−1.A simple model treats the main galaxies as point massesmoving along a linear orbit—an ellipse of zero width. Inthis approximation, the two galaxies coincided at the bigbang t0 13 Gyr ago, separated smoothly with the Hubbleflow, reached a maximum separation of ∼1 Mpc some5 Gyr ago and are now falling together. To account forthis history, the LG’s total mass must be about 4 ×1012Mand its mass-to-light ratio M/LV 100. This is severaltimes most estimates of the MW’s M/L ratio, a sign thatunseen dark matter is important in extragalactic dynamics.Thus it seems that much of the LG’s mass resides instructures larger than individual galaxies. CosmologicalN-body simulations show that gravity deforms a smoothdistribution of dark matter with a plausible spectrum ofdensity fluctuations into a complex web of ‘pancakes’ andfilaments; this web constantly evolves as ever-larger scalesbreak away from the Hubble expansion. Halos of darkmatter form at the intersections of filaments and grow asmass flows along the filaments. In this picture, the MWand M31 are presumably linked by a major filament andare ‘sucked together’ as this filament collapses.Spins and swingsIn gravitational clustering, galaxies acquire their spinsfrom tidal torques. At early times, the material whicheventually forms a galaxy has an irregular shape, andas density fluctuations grow this material feels a torquedue to the tides of other proto-galaxies. Groups seemnatural places to apply this scenario; for example, it hasbeen argued that the MW and M31 are mutually more orless edge-on because each provided the tidal torque whichspun up the other. The further assumption that the totalspin and orbital angular momentum of the LG is zero thenyields a definite prediction for the M31’s transverse motionwith respect to the MW.This simple calculation is less plausible on closerinspection. First, in a tally of angular momentum, spinsare less important than satellite orbits; for example, theLARGE MAGELLANIC CLOUD’s orbit about the MW has severaltimes the angular momentum of the MW’s spin, andthe same may be true for M31 and M33. Second, theexternal torque on the MW–M31 binary due to the nearbygalaxies shown in figure 1 is sufficient to give the paira transverse velocity of at least ∼40 km s−1. This orbitalswing completely dominates the total angular momentumof the LG; clearly the LG’s dynamical isolation is veryimperfect. It is possible that MW and M31 set each otherspinning, but the reflex torque on their relative orbit is tinycompared with the torques due to surrounding galaxies.Tests of the tidal torque hypothesis based on auditing theangular momentum of the LG seem doomed to founderon this complication.Copyright © 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, UK1Groups of GalaxiesENCYCLOPEDIA OF