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 $N$-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