Dynamics in Elliptical Galaxies

Dynamics in Elliptical Galaxies - Dynamics in Elliptical...

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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 v/ σs of rotation velocity to (line-of-sight) velocity dispersion. For a given shape of potential, v/ σ 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
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