| We 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 collisionless 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 snowplow 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 confirmed 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. |
