Title

Random interactions with disk galaxies

Date of Award

1996

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

Advisor(s)

Geoffrey Fox

Keywords

gravitational interactions, velocity perturbations, Astronomy, Astrophysics, Computer science

Subject Categories

Astrophysics and Astronomy

Abstract

Numerical simulations of galaxies have been used to study numerous characteristics and interactions of elliptical galaxies while disk galaxies have not been studied extensively with fully three-dimensional models of all components. We use a parallel implementation of the hashed oct-tree algorithm to study gravitational interactions of a disk galaxy like our Galaxy with companion galaxies. We simulate the encounter presently occurring between a dwarf galaxy recently discovered in Sagittarius and our Galaxy. A model that reproduces the observed position, size, velocity, proper motion, and velocity gradient of the dwarf in its likely pre-disk encounter and an alternative post-disk encounter scenario are studied. For the pre-collision case, the history of the apparent orbit, previous disk crossings, and associations with nearby globular clusters are discussed. Gas stripping during disk crossings and observed high dark matter fraction for most dwarfs have implications for the composition of dark matter. We also study the effect of an orbiting intermediate-mass companion such as the Magellanic Clouds system (Large and Small Magellanic Clouds) on a disk galaxy like the Milky Way. The model demonstrates that a companion with a mass fraction of 0.075 can generate height and perpendicular velocity perturbations of the primary galaxy's inner disk that are on the order of several hundred parsecs and $\sim$10 km s$\sp{-1}$, respectively. The companion also significantly deforms the dark-matter halo surrounding the Galaxy. The small perpendicular motions generated by the companion may account for some of the vertical deformations or corrugations that are observed in the Milky Way gas disk and for the slightly elevated velocity dispersions of the gas in some weakly interacting face-on galaxies. Next generation simulations must tackle both gravitational particle and hydrodynamics properties of galaxies. We discuss how algorithms which model these two components can be merged. The similar hierarchical tree-structures developed for multipole particle simulations and adaptive mesh continuum simulations provide a consistent formalism with which to unite these two classes of solvers. These hierarchical techniques naturally develop from the underlying locality of the discrete operator describing these interactions. This locality can be exploited in parallel implementations of such algorithms.

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