VERTICAL OSCILLATION OF THE GALACTIC DISK

The local structure of the Galactic disk is similar to that of a self-gravitating one-dimensional slab. Such a slab may have discrete oscillation modes. This implies that a disk will not necessarily reach a new time-independent equilibrium state after a perturbation but may exhibit undamped oscillation. The possible oscillation modes and their frequencies are derived for three families of one-dimensional disk models with and without external halos. In the presence of a halo, the lowest order mode is a sloshing or seiche mode and corresponds to oscillation of the entire slab in the halo background potential. This oscillation is undamped and may be excited to large [O(1)] amplitudes. Symmetric breathing modes predicted by the linear theory may be excited to 3% amplitude before their natural widths are sufficient to cause the mode to damp. Higher order modes damp at lower limiting amplitudes. I also consider modes with horizontal as well as vertical structure. The lowest order mode generalizes to a bending wave which is weakly damped for long wavelengths. The lowest order seiche mode was easily excited in a n-body simulation by passing projectiles (star clusters, dwarf galaxies), suggesting that the effects of this mode might be observable in the Galactic disk. In particular, it is possible that the associated long-wavelength bending modes, as excited by the Magellanic Clouds, for example, are related to the Galactic warp. Attempts to produce the higher order oscillations failed. Although it is possible that higher order modes may exist in galactic disks, they do not appear to be easily excited by a large-scale perturbation. The effect of small-scale perturbations has not been investigated. Implications for the often-used isothermal approximation are discussed.