Field of work of Diploma thesis
Jan-Uwe Ness
at the 
Institut für Theoretische Physik
und Astrophysik
Christian-Albrechts Universität Kiel
Leibnizstraße 15 - Physikzentrum
24118 Kiel

Simulation of interacting galaxies with starformation
,,Minor merger'' (= interaction of a small satellite galaxy with a massive disk galaxy)

In a galaxy as our Milky Way each individual star moves about according to the combined gravitational force from all other stars. From these gravitational forces the courses of the stars can be calculated. The computational efforts for doing this are tremendous, since for N stars N2 equations have to be solved. For this reason a Galaxy actually consisting of 100 billion stars is represented by only few 10 thousend testparticles. Measures are taken to compensate for the "granularity" resulting from the reduced number of particles.
In my project I put up a system of testparticles with initial positions and velocities as found in our Galaxy. At first I tested whether this system is stable over a sufficient amount of time (i.e., 1.5 Gyrs). This stable model was now exposed to a minor merger event, i.e., a second much smaller (spherical) galaxy was placed on the brink of the larger disc galaxy model with an initial velocity corotating with the host galaxy and with an additional vertical velocity component. The following pictures show how the structures of the host galaxy and of the satellite galaxy are influenced by the interaction.
1) Development of the structure
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2) Development of the structure of the satellite galaxy

The development with time of the Lagrange-radii for 0.1%, 0.5%, 1%, 5%, 10%, 20%, 50%, 75%, 90% and 95% of the total mass of the satellite galaxy and the distance to the rotation axis. The squares mark time steps of 150 Millionen years, respectively.

The distance of the satellite galaxy to the rotation axis of the disk galaxy is hardly reduced during the interaction, but reduces quite rapidly in the last two timesteps. While sinking towards the center of the disk galaxy the satellite galaxy dissolves.
3) Development of the disk structure

Plot of the development with time of the half mass height for the outer regions and the center

The vertical structure of the disk changes due to the interaction. While the inner region doesn't change much, the disk becomes significantly thicker in the outer regions.

4) Development of the gas dynamic

The dynamic of the gas component is simulated by treating some particles as ,,gas particles''. These particles have the property of sticking together, thus forming one particle out of two (,,sticky particles''). This merging of particles is a process which needs a collision, but only collisions between particles moving at small relative velocities end up in such a merger process. One therefore distinguises between the collision rate and the merger rate, the number of particle collisions/mergers per time interval.
In my thesis I added the process of disruption of particles which suffered from many mergers, thus beeing quite massive. These particles disrupt on time scales depening on their mass (which can increase due to further merges, while ,,waiting'' for their disruption, but the time scale then further decreases). Physically this process originates from star formation in massive clouds, and the cloud material gets driven away by stellar radiation and supernova explosions. I therefore call the rate connected with these disruptions starformation rate.

Development of the starformation rate (solid line), the merging rate (dotted line) and the collision rate (dashed line).  For comparison the rates are normalized to their starting values.

The collision rate of gas particles remains constant. The merging rate and the starformation rate, which is strongly correlated with the merging rate, increase at first to a constant level, but decrease due to the enhanced volume as a consequence of the interaction and thus the smaller collision probability.
Implementation of star formation as fragmentation of large gas clouds prevents the gas component from becomming too clumpy, which would lead to a decrease of the collision rate. The constant level shows that star formation keeps the gas component stable for at least 2Gyr.

Diploma thesis(Postscript):(German only)

Interaction of galaxies with different sizes
star formation induced by the interaction

photographed by Dr. Uwe Schwarzkopf, 8. Apr.1997
DFOSC at the 1.54-m Danish Telescope at La Silla (Chile),
Exposure time: 30 min in the r-Band.

Introduction(in english)
Summary(in english)


last modified: Friday, 14-Nov-2008 21:39:03 CET MET