Simulation

A videomovie, showing a simulation of a disc galaxy perturbed by a companion, has been made. It is easy to see some time-dependent phenomena in the movie that would have been hard to discover on paper plots.

The videomovie shows results from a 2-dimensional N-body simulation with 60 000 particles using a polar coordinate code. The disc is self-gravitating and surrounded by an inert spherical halo. It has Mestel's density distribution, which gives a flat rotation curve, and the companion is represented by a point mass which passes on an initially parabolic orbit.

The initially stable disc evolves as follows: A very sharply defined material arm quickly appears. After some time a counter arm is formed due to self-gravitation. Soon after the appearance of the counter arm, a third arm forms from the remains of the first material arm. This third arm has a higher radial velocity than the counter arm which makes it pass, or expand, through the latter. Some time later, after the passage of the companion, a density wave pattern, much more long lived than the material arms, is formed. The density waves first appear between the material arms giving the impression of a fork in the arm, such as has been observed in several galaxies. As the material arms are decaying when this happens, a fork would be a short lived phenomenon.