The propagating star formation model with anisotropic probability distribution is investigated. In each star-forming site we define the probability ellipse and show that its two parameters, the excentricity and the orientation relative to the galactic rotation, are closely related to the thickness and inclination of the resulting spiral arms. The relative size of a star-forming region with respect to the whole galaxy is also discussed. Simulations are compared to the observed galactic morphologies and we mimic the differences between the two groups of galaxies of types M101 and NGC 7217.

Propagating star formation

The idea that star formation at one place in a galaxy can initiate star formation in its neighbourhood was first suggested by Öpik (1953) and Oort (1954). Since then, a possible chain of physical processes that joins two regions of subsequent star formation has been proposed in which ionizing radiation from massive stars in a cluster leads to the disruption of the parental molecular cloud via supersonic champagne flows halting further star formation. The mechanical energy input from stellar winds and supernova explosions causes the agglomeration of gas in expanding shells. Their fragmentation, the building of molecules in high opacity areas, and large-scale gravitational instabilities may produce molecular clouds, where the next generation of star formation occurs.

The star-forming cycle described above is the basis of deterministic PSF models (Palouš et al. 1994). However, the physical parameters such as density, metallicity and cooling times of the ISM, are only partly known.