4 research outputs found
Development of Realistic Simulations of the Interactions Between Stars and the Interstellar Medium in Disk Galaxies
fWe have developed GALAXY, a two dimensional, self-consistent N-body plus hydro-dynamic computer program to model and study the behavior of disk galaxies like our Milky Way. Our goal is to better understand how such galaxies maintain their spiral structure for billions of years.
The program utilizes two independent gravitating N-body components. One simulates the collision-less star particles in the galaxy, and the other simulates colliding clouds. A gravitating hydrodynamic code simulates the interstellar medium. Additionally, a static spherical halo and a central black hole interact gravitationally with the other components of the model galaxy. All components interact with each other through various evolution processes and gravity, and they exchange mass, energy, and momentum.
For this dissertation I have primarily studied these interactions between the galaxy’s individual components. These interactions include cloud formation from gas by Jeans instability, star formation through cloud collisions, star formation due to the snow-plow effect, gas infall from the halo, and supernovae. Each process is controlled by one or several parameters. These parameters are specified at the start of a simulation, along with other parameters which define the initial setup of the model galaxy. The influences of these evolutionary processes, and of different initial conditions of the model galaxy, were probed through series of simulations, during which all parameters were systematically varied. The simulations showed that the stellar evolution processes of the program have the ability to trigger the formation of spiral features in galaxies.
Through simulations involving galaxies with counter-rotating components we con-firmed analytically derived predictions about such galaxies, like the formation of a leading one-armed spiral, the weakening of this spiral as Toomre’s Q of the system exceeds 1.8, and the strengthening of this spiral as the fraction of counter-rotating components increases.
In another series of simulations we explored the properties and the origin of lopsided galaxies as a result of the high speed passage of a companion galaxy. We found that the disk’s center of mass spiraled inward, creating observed or observable phenomena, including one-armed spirals, clumps of particles, and the possible suppression of a central bar
One-Armed Spiral Waves in Galaxy Simulations with Counter-Rotating Stars
Motivated by observations of disk galaxies with counter-rotating stars, we
have run two-dimensional, collisionless N-body simulations of disk galaxies
with significant counter-rotating components. For all our simulations the
initial value of Toomre's stability parameter was Q = 1.1. The percentage of
counter-rotating particles ranges from 25% to 50%. A stationary one-arm spiral
wave is observed to form in each run, persisting from a few to five rotation
periods, measured at the half-mass radius. In one run, the spiral wave was
initially a leading arm which subsequently transformed into a trailing arm. We
also observed a change in spiral direction in the run initially containing
equal numbers of particles orbiting in both directions. The results of our
simulations support an interpretation of the one armed waves as due to the two
stream instability.Comment: 13 pages, 4 figure
Sloshing in High Speed Galaxy Interactions
Observations of lopsided spiral galaxies motivated us to explore whether the
rapid passage of a companion galaxy could cause them. We examine whether the
center of mass of the visible matter becomes displaced from the center of mass
of the dark halo during the intruder's passage, thereby causing an asymmetric
response and asymmetric structure. Two dimensional -body simulations
indicate that this can happen.
We also explore some consequences of this offset. These include the center of
mass of the visible disk following a decaying orbit around the halo center of
mass and the development of transient one-armed spirals that persist for up to
six rotation periods.
We then study the results of a variety of initial conditions based on such
offsets. We report on the results of several runs in which we initially offset
a disk from its halo's center of mass by an amount typical of the above
interaction. In some runs the halo is free to move, while in others it is held
fixed. We used three different mass distributions for the halo in these runs.
We find that the disk's center of mass spiraled inward creating a variety of
observed or observable phenomena including one-armed spirals, massive clumps of
particles, and counter-rotating waves. The systems settle into relatively
axisymmetric configurations. Whether or not the end states included a bar
depended on a variety of initial conditions.Comment: 20 text pages, 3 tables, 24 figures. A gzipped postscripped version
with higher resolution figures can be downloaded from
http://butch.umephy.maine.edu/kickers/Research/Sloshing/ . Accepted for
publication in The Astrophysical Journa
INTERACTIONS BETWEEN STARS AND THE INTERSTELLAR MEDIUM IN DISK GALAXIES
2000 We have developed GALAXY, a two dimensional, self-consistent N-body plus hydrodynamic computer program to model and study the behavior of disk galaxies like our Milky Way. Our goal is to better understand how such galaxies maintain their spiral structure for billions of years