8. Galactic rotation and force
Flat disk galaxies rotate rapidly and the rotation influences the shapes of expanding shells: in the galactic plane, shells are not spherical any more but elliptical. In agreement with this result, many HI holes discovered in nearby galaxies (Brinks, 1994) have elliptical shapes. Due to galactic shear, the mass accumulated in the shell slides to the tips. This changes the fragmentation conditions in the galactic plane forming instability regions around the tips of elongated shells near the symmetry plane of the galaxy.
The plane parallel component of the gravitational acceleration is derived from the galactic rotation curve. We adopt the rotation curve proposed for the Milky Way by Wouterloot et al. (1990)
where is the galactocentric distance, 8.5 kpc, is the linear velocity of rotation and = 220 km/s.
The component of the galactic gravitational acceleration is approximated by a formula (Kuijken & Gilmore, 1989)
where is the halo density, and are the surface density and scale height of the stellar disk near the Sun.
The results of our 3D computer simulations with the galactic rotation given by formula (27) and the component of the force perpendicular to the galactic plane given by formula (28) in the multi-component disk are shown as the time sequence of snapshots in Fig. 7. The instability develops only in the regions near the elliptical tips at z close to 0. All the rest of the expanding shell is stable.
© European Southern Observatory (ESO) 1997
Online publication: March 24, 1998