Our results provide a first insight into the required conditions for the development of Elmegreen-Vishniac instabilities within the expanding and decelerating shells giving constraints on the propagation of star formation. The density gradients, the Kz force and the galactic shear distorting the expanding and decelerating shells from spherical symmetry are considered in computer simulations. These effects can enhance the fragmentation in the galactic plane and at the tips of elliptical shells, and supress it at other places. The newly formed clouds, possibly gravitationally stable at the time of their formation, may become unstable at later times and transform themselves into new places of star formation. Deviations from our results may be due to preexisting inhomogeneities, streaming motions connected with the spiral arms and small-scale density gradients.
Fragmentation of expanding shells may be one of the mechanisms how the star formation propagates in galaxies. In dwarf galaxies similar to LMC, SMC or Holmberg II, the conditions for fragmentation are fulfilled in almost any part of the expanding shell, therefore, the star formation can easily propagate. In the Milky Way-type spiral galaxies, where the fragmentation is restricted only to a rather narrow strip near the galactic plane, the star formation can propagate due to the fragmentation of expanding shells only into a small fraction of the interstellar space. At large heights above the galactic disk the fragmentation is almost prevented. In these galaxies the star formation is related to other large-scale mechanisms such as spiral density-waves, bars, galaxy mergers and interactions of galaxies.
© European Southern Observatory (ESO) 1997
Online publication: March 24, 1998