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Galaxies and the Universe - Dynamics in Elliptical Galaxies

Galaxies and the Universe - Dynamics in Elliptical Galaxies...

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1/15/12 Gala[ieV and Whe UniYeUVe - D\namicV in EllipWical Gala[ieV 1/4 ZZZ.aVWU.Xa.edX/keel/gala[ieV/ellipd\n.hWml Dynamics in Elliptical Galaxies For decades it was considered obvious beyond the need for observations that elliptical galaxies were oblate objects, rotationally flattened. The observations to actually check this were quite difficult, since ellipticals don't have convenient numbers of H II regions, but revealed that most ellipticals rotate far too slowly for their apparent shapes - the shape must be an intrinsic property of the stars' velocity distribution (see Davies et al. 1983 ApJ 266, 41). An important discriminant is the ratio Y/ ıs of rotation velocity to (line-of-sight) velocity dispersion. For a given shape of potential, Y/ ı predicts flattening ε. Elliptical galaxies mostly ignore this prediction, leading to the notion that their shapes are triaxial ellipsoids supported not by rotation, but by anisotropy in the velocity distribution of the stars. This is shown in Fig. 3 of Davies et al. (shown courtesy of the AAS): To model these galaxies in detail, one must consider statistical mechanics of available kinds of orbits (tube, box...) and their stability - see Binney and Tremaine ch. 2-5). In general, one expects orbits in such a family (in either physical or phase space) to obey the ergodic theorem , so that a star will eventually approach arbitrarily close to any specified point within the envelope of its orbital family. This approach of loading orbit families, to get a self-consistent set of orbits and potentials, has worked better for shapes than M/L ratios. The behavior of ı (r) at small radii contains information on the degree of central mass concentration (is there a supermassive black hole?), but even here anisotropic velocity dispersion can complicate the analysis, as seen in the controversy over the core of M87. Sargent et al. 1978 (ApJ 221, 731) and Young et al. 1978 (ApJ 221, 721) announced with
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