Stochastic star formation and spiral structure of galaxies.

A model of self-propagating star formation (SPSF) in a two-dimensional differentially rotating disk is examined in order to determine its suitability as a model for real galactic evolution and the important parameters for ascertaining the features of known galactic structure. The model is rendered stochastic insteady of completely deterministic by introducing a finite probability for star formation, and the discrete array selected to represent a galaxy is extended to a size large enough to ensure that finite-size effects do not dominate the results. The analysis indicates that stochastic SPSF produces large-scale spiral features that are stable over long times of the order of the lifetime of a galaxy and that these features, given by the loci of star formation, rotate 'quasi-rigidly'. The spiral structure obtained using the observed rotation curves for M81 (Sb) and M101 (Sc) are shown to correspond very well with the appearance of the arms in these galaxies (e.g., pitch angle and arm density). It is found that each morphological type appears to be due simply to the rotation curve that exists in the galaxy; i.e., there is no evolution between morphological types in this model.