Effect of Population II Stars and Three-Dimensional Motion on Spiral Structure

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Abstract
A three-dimensional computer model of a fully self-consistent isolated disk galaxy of finite thickness is described using approximately 50 000 simulated Population I stars. Population II stars are represented by a fixed imposed gravitational field. This model is used to study the stability of a thin-disk galaxy for varying fractions of Population II stars. Relatively long-lived two- and multi-arm spirals are observed when the proportion of Population II stars is large. We compare the behaviour of the three-dimensional model, in which motion of the Population I stars out of the plane is allowed, and the behaviour of the simpler two-dimensional model, in which the motion of the stars is confined to the plane of the disk. The lifetime of the spirals is significantly longer in the more realistic three-dimensional model. A long-lived density-wave spiral is observed lasting, without significant change of shape, for at least 10 rotations, in the presence of strong differential rotation.

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