eaa.iop.orgDOI: 10.1888/0333750888/1672 Elliptical GalaxiesRoger L Davies FromEncyclopedia of Astronomy & AstrophysicsP. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888Downloaded on Thu Mar 02 23:07:24 GMT 2006 [131.215.103.76]Institute of Physics PublishingBristol and PhiladelphiaTerms and ConditionsElliptical GalaxiesENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSElliptical GalaxiesElliptical galaxies are smooth, quiescent star-piles, devoidof any of the spectacular structures found inSPIRALGALAXIES.They have no disk, no spiral structure and only smallamounts of the gas and dust. As a result there are noobvious symptoms of continuing star formation: no H IIregions or young star clusters. They are the simplestgalactic systems comprising just a single component,relatively bright in the center but fading rapidly withincreasing radius. Elliptical galaxies are found mostlyin the denser regions of the universe, from rich clustersto small groups; truely isolated ellipticals being relativelyrare. Figure 1 shows the central region of the Coma clusterwhich is dominated by a population of ellipticals. Themost luminous elliptical galaxies are amongst the brightestgalaxies we know of and this, combined with theirapparent simplicity, has made them targets of detailedstudy, not only so that we can understand their ownevolution but also so that they can be used as standardcandles to determine distances.The simple structure of elliptical galaxies is reflectedin their place inHUBBLE’S CLASSIFICATION. They are character-ized by a single number, the ellipticity ε = 10(1 − b/a),where b and a are the projected angular extent of the shortand long axis of the galaxy on the sky. E5 galaxies havea long axis twice that of the short axis and are almost theflattest ellipticals. Some galaxies have been classified E6and E7 but these have almost always been found to har-bor stellar disks. Because the classification depends onthe projected axial ratio it is not a physical classification.By making the simple assumption that ellipticals have thetwo long axes equal with the third axis shorter (i.e. theyare oblate, like a tangerine) Hubble was able to invert theobserved distribution of projected shapes to estimate thetrue distribution. He estimated that on average ellipticalshave a short axis about 65% of the length of the long axiswith a dispersion of about 15%. This picture remained cur-rent until the late 1970s but has been dramatically revisedsince then. In recent years attempts to produce a physicalclassification for ellipticals, and new ways of estimatingtheir intrinsic shapes, have been developed (see later).In this article I will limit myself to a discussion ofclassical elliptical galaxies. These range from the brightestgalaxies, which are about 40 times as bright as the MilkyWay, to dwarf galaxies that are 100 times fainter than theMilky Way and that are typically found as companionsto more massive galaxies (e.g. M32, the companion tothe Andromeda nebula, M31). I will not includeDWARFSPHEROIDAL GALAXIESwhich are sometimes, erroneously,refered to as dwarf ellipticals. Luminous elliptical galaxieshave roughly the same colors as K-giant stars giving thema yellow-orange hue; less luminous galaxies have slightlybluer colours. In appearance and spectral characteristicsthe bulges of spiral galaxies resemble low-luminosityelliptical galaxies and historically the bulge component ofM31 was taken as typical of ellipticals. There are manyFigure 1. The center of the Coma cluster of galaxies, dominatedby the bright elliptical galaxy NGC 4874, which is surroundedby a swarm of fainter, featureless elliptical galaxies. (Imagecourtesy of the Isaac Newton Group of Telescopes, La Palma andthe SMAC (Streaming Motions of Abell Clusters) team.)parallels between ellipticals and bulges and I will compareand contrast them throughout this article.In the next section I will draw together the ways inwhich we have determined the distribution of light andmass as a function of radius within elliptical galaxies,including a discussion of attempts to refine our viewsof the intrinsic shapes of elliptical galaxies and attemptsto detect their massive dark halos. I follow that witha discussion of the special properties of elliptical galaxycores, where we often find separate components, evenrotating in the opposite direction to the bulk of the galaxy,will be discussed. I also discuss the extensive searchesfor black holes in the centers of ellipticals. Then I describehow we learn about the evolution of the stars that make upellipticals and how the large-scale properties of ellipticalgalaxies scale with each other: luminosity, mass, surfacedensity, color, heavy element abundance and rotation.Finally I bring together all of the facets of elliptical galaxiesand present the current thinking about how ellipticalgalaxies formed and evolved.Structure, size, luminosity and massElliptical galaxies are so called because the contoursof constant intensity (called isophotes) are concentricellipses. This is a very good description of most ellipticalgalaxies but there are small but significant variations fromthis simple form. High-quality images show (a) systematicchanges in ellipticity with increasing radius, the mostcommon form being a steady increase in the flatteningand (b) that the position angle of the major axis of theisophotes changes with increasing radius. In most casesthese changes are small (one or two ellipticity classes or afew degrees) but in a small minority they can be dramatic.The sizes of ellipticals are measured by fitting ananalytic form to the fall in brightness (averaged inCopyright © 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, UK1Elliptical GalaxiesENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSconcentric circular annuli) with increasing radius: theluminosity profile. The most commonly used form wasproposed by Gerard de Vaucouleurs in 1948 on the basisof images taken on photographic plates. He proposedI(R) = Ieexp{−7.67[(R/Re)1/4− 1]}= Ie10{−3.33[(R/Re)1/4−1]}where Re, the effective radius, is the radius that containshalf the total light of the galaxy and Ieis the surfacebrightness (the amount of light from a square arc secondof the galaxy) at Re. This law can be integrated to givea finite total light of Itot= 7.22πRe2Ie. de Vaucouleurs’law, as it is known, fits remarkably