Physics
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........31s&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF WASHINGTON, 1995.Source: Dissertation Abstracts International, Volume: 56-12, Section: B, page: 68
Physics
1
Black Holes, Draco, Ursa Minor
Scientific paper
I examine the dynamics of dwarf spheroidal galaxies and the thick disk globular clusters. I argue for models that require no special or exotic conditions. The dwarf spheroidal's large dark matter densities can be explained by ordinary large black holes with masses predicted in the standard model for active galaxies. The dynamics of the thick disk globular clusters can be explained by well understood dynamical friction processes. I explore the evidence for massive black holes at the cores of Local Group galaxies and provides evidence for a massive black hole (10-20 million solar masses) lying at the center of each of the two galaxies with very large central mass densities--Draco and Ursa Minor. Such black holes produce the large observed velocity dispersions and create a density and distinctive velocity cusp close to the center. Star counts reaching the main sequence of Draco and Ursa Minor will lead to more accurate centering of the isopleths and should reveal a more prominent cusp. I investigate the evidence for disk heating (the increase of the velocity dispersions of the disk stars with time) and some implications of the disk heating models on the kinematics of the thick disk globular cluster system. The newest observations of the age-velocity relation for the solar neighborhood show the exponent of time (p) in the age-velocity relation to be p = 1/2. Such a dependence of the velocity dispersion with time could be explained by either a dark matter halo model consisting of massive black holes or the accretion of satellite galaxies. An implication of the disk heating models is that the thick disk globular clusters should have a much smaller scale height than what is observed. In the standard models of disk heating, the oscillations of these clusters should be significantly damped after a Hubble time because of dynamical friction. The fact that the disk globular clusters have kinematics similar to the thick disk stars violates this expectation. The globulars could be heated by the same mechanisms that heat the disk stars. I estimate the amount of heating needed by simulating the dynamical friction acting on the globulars. Simulations of dynamical friction acting on a distribution of massive particles initially with thick disk kinematics are used to show that dynamical friction can explain the observed kinematics of the thick disk globular clusters. There is no need to invoke extra heating of the globular clusters by whatever heats the disk. Since dynamical friction preferentially destroys objects on nearly circular orbits that stay close to the plane, the globular clusters remaining today are the ones that were in the high energy tail of the initial distribution.
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