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Tri-Axial Dynamics in the Core of Normal Galaxies

Published online by Cambridge University Press:  30 March 2016

M. Schwarzschild
M. Schwarzschild*
Affiliation:
Princeton University Observatory, Princeton, New Jersey 08540, USA

Extract

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Please let me for fifteen minutes completely ignore all gas in the cores of galaxies. Clearly the core gas exhibits fascinating, varied and consequential phenomena. Still it would seem to me a plausible assumption that while the gas might be decisively affected by the stars, this gas might not have major effects on the stars, at least in their dynamical behavior. Accordingly I shall here consider purely stellar systems.

Obviously in the core of a galaxy, for dynamical consideration we have to distinguish sharply between the nucleus of a galaxy and the larger components of the galaxy co-occupying the core. Following Baade’s usage the nucleus is a stellar system of at most a few parsecs in radius and of a density substantially higher than the central densities of the other components. That the nucleus of a galaxy, at least in its equilibrium state -though not necessarily in its origin- is a distinct, self-contained stellar system seems to me strongly indicated by the rotation velocity measurements in the nucleus of the Andromeda Nebula (Lallemand et al. 1960; Walker, 1974) as well as by the photometric profile determination of the same nucleus (Light et al. 1974). For a nucleus of 108 stars the relaxation time for stellar encounters is, according to Spitzer (1971), comparable to a Hubble time while stellar collisions are ignorable. On the other hand, for a nucleus containing 1010 stars, which may be the right order of magnitude for more pronounced nuclei, the relaxation time may be comfortably long enough so that encounters may be ignored, but the frequency of collisions is not ignorable. In both cases therefore it would seem that in the centermost portions of nuclei of galaxies either encounters or collisions play a decisive dynamical role but the same may not be true in the outer portions of these nuclei. For the larger subsystems occupying the core of a galaxy, obviously neither encounters nor collisions play a significant role. To stay within the limits of my knowledge, I shall limit myself here to encounterless and collisionless stellar systems, a marginal approximation for the nuclei proper.

Type
Joint Discussion
Information
Highlights of Astronomy , Volume 5: Highlights of Astronomy. As presented at the XVIIth General Assembly of the IAU, 1979 , 1980 , pp. 205 - 208
Copyright
Copyright © Cambridge University Press 1980

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