eaa.iop.orgDOI: 10.1888/0333750888/2142 Galaxy EvolutionMauro Giavalisco FromEncyclopedia of Astronomy & AstrophysicsP. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888Downloaded on Thu Mar 02 23:13:08 GMT 2006 [131.215.103.76]Institute of Physics PublishingBristol and PhiladelphiaTerms and ConditionsGalaxy EvolutionENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSGalaxy EvolutionThe galaxies that we observe in the universe at the presenttime exhibit a remarkable variety of properties, such asmorphology, colors, luminosity and dynamics. There aregalaxies which are one hundred times brighter than ourown Milky Way and others that have one thousandth itsluminosity. Some galaxies have very regular shapes, otherare amorphous. There are galaxies with spheroidal shapethat have little amount of rotation, and whose stars havelarge ‘thermal motions’ (like the molecules of a hot fluid),and galaxies that resembles rotating disks, where self-gravity and centrifugal force balance each other. Thereare galaxies with high amounts of gas (mostly hydrogenand helium) and dust, and a vigorous activity ofSTARFORMATION. Other galaxies consist mostly of old, evolvedstars, indicating that the activity of star formation in thesesystems ended in a remote past.Why do galaxies show such a variety of properties?How could they form and diversify in this way, and whendid they do so? Is a given property an indication of aspecific evolutionary phase in the life of a galaxy, or havegalaxies with different properties formed and evolvedin different ways? Which physical mechanism is eachproperty the manifestation of?Such questions are among the most important in thestudy of cosmology, and yet they still elude a consistentand complete explanation. There was a time duringthe cosmic evolution that followed theBIG BANG whenthere were no galaxies, and the universe was filled withmatterand radiationuniformly distributedthroughoutthespace. Only very tenuous perturbations of the densityof matter and radiation altered this otherwise almostperfect homogeneity. Then the universe underwent atransition from this phase to the highly structured onethat characterizes the current epoch.GALAXY FORMATION andevolution are the results of a complex sequence of eventsthat occurred during this transition. Their importancerests in the fact that the chronology and dynamics of theseevents are intimately linked to the global cosmologicalproperties of the universe (i.e. the age of the universe,if it will expand for ever or if it will eventually collapseagain in a Big Crunch, or if the expansion is an acceleratedone or not) and to those of the matter that fills the cosmicspace (e.g. the composition of the dark matter and itsabundance relative to the ordinary, visible matter). Thegoal of galaxy evolution is to reconstruct back in time thephysical mechanisms that led to the present-day galaxiesand to explain them in terms of the properties of the matterand of the cosmology.The morphology, structure and luminosity ofgalaxiesThe galaxies in the present-day universe have beenextensively studied for more than seventy years and, asa result, we have now a fairly accurate knowledge of theirphysical properties, such as the distribution of luminosity,abundances, morphology, age, chemical composition,Figure 1. The Hubble morphological classification. Ellipticalgalaxies are ordered from E0 to E7, according to their ellipticity.The S0 type essentially represents elliptical galaxies that alsohave a disk. Spiral galaxies are divided into normal and barredtypes, and are ordered from the ‘a’ to the ‘d’ sub-types(including mixed type such as ‘bc’ and ‘cd’ not shown in thefigure), according to the increasing importance of the diskcomponent over the bulge. Irregular galaxies are not shown.(Photo credit: STScI OPO.)and internal kinematics. This knowledge forms thebackground against which observations of the propertiesof galaxies at earlier cosmic epochs are compared to testand quantify galaxy evolution.Galaxy morphologyThe morphology of the luminous component of galaxies,namely stars, gas and dust, is probably the most obviousmanifestation of the diversityof galaxy properties. Duringthe first half of the 19th century,EDWIN P HUBBLE classifiedgalaxy morphologies into the four principal types ofellipticals, lenticulars, spirals and irregulars. Figure1 shows the classical representation of the Hubbleclassification, also referred to as the Hubble sequence.Essentially, theHUBBLE CLASSIFICATION scheme is basedon the relative proportions of the two major structuralparts of the luminous component of galaxies, namelythe bulge and the disk. The bulge is generally made ofrelatively old and evolved stars, which are characterizedby red colors, and is shaped like an ellipsoid with variousdegrees of flattening and oblateness (ellipsoids with noflattening and oblateness are spheres). In general thebulge contains no appreciable amount of dust and gas,and therefore has no activity of star formation. The diskis composed by a mixture of old and young stars and bygas and dust, and it is frequently site of star formationactivity. Its colors are generally bluer than those of thebulge. The most visually striking feature of the disk is thefrequent presence of the‘SPIRAL ARMS’, density waves thatgenerate in the disk and which contain active regions ofCopyright © 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, UK1Galaxy EvolutionENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSstar formation. Other articles in this encyclopedia discussgalaxy morphology in greater detail.Bulge and disk do not only differ for the compositionof their stellar populations and interstellar medium, butalso for their dynamics. Galaxies are large concentrationsof mass, and they would collapse under the effect oftheir own self-gravity if some opposing force were notthere to keep them in dynamical equilibrium. Disksare kept in equilibrium by rotation, which provides thecentrifugal force to oppose gravity. Bulges have modestor no rotation, and are sustained against gravity by the‘velocity dispersion’ of their stars, which is similar to themotions of molecules in a hot gas. These motions providethe pressure that contrasts gravity and keeps the structurein dynamical equilibrium.ELLIPTICAL GALAXIES consist only of the bulge compo-nent and have no disk.LENTICULAR