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Astron. Astrophys. 328, 121-129 (1997)

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8. Galactic rotation and [FORMULA] 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 [FORMULA] is the galactocentric distance, [FORMULA] 8.5 kpc, [FORMULA] is the linear velocity of rotation and [FORMULA] = 220 km/s.

The [FORMULA] component of the galactic gravitational acceleration is approximated by a formula (Kuijken & Gilmore, 1989)


where [FORMULA] is the halo density, [FORMULA] and [FORMULA] 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.

[FIGURE] Fig. 7. The time sequences of shell expansion including the galactic differential rotation, component of force perpendicular to the galactic plane in the multi-component disk. The snapshots are for t = 10, 20, 30, 40 and 50 Myr, a black color shows regions where the value of fragmentation integral [FORMULA] = 0, a white color shows regions where the value [FORMULA]. The black line has a length of 1 kpc.
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© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998