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Session 68 - Disk Galaxies.
Display session, Wednesday, January 15
Metropolitan Ballroom,
When a galactic disk is tilted with respect to a flattened dark halo, it precesses and feels dynamical friction from the the gravitational wake induced in the responsive halo. When the halo is non-rotating or rotating in the same sense as the disk, dynamical friction has a damping effect which tends to bring the disk and halo into alignment. However, when the halo is counter-rotating with respect to the disk dynamical friction acts with the opposite sign exciting a tilt in a disk leading to an instability which can dramatically affect the evolution of the disk (Nelson amp; Tremaine 1995). The same processes apply to galactic rings as seen in the polar ring galaxies. In this poster, we present a series of N-body and hybrid N-body/N-ring simulations of tilted disks and rings which demonstrate the damping and excitation mechanisms described above from halo dynamical friction. Galactic warps are a natural consequence of either a disk settling into the halo plane or being excited by a counter-rotating halo. The time scale for damping is small, however, suggesting external perturbations such as infalling satellites are necessary to tilt the disk with respect to its halo. Dynamical friction also has a strong damping effect on the evolution of a galactic ring when the ring is at intermediate inclinations. Only rings formed in the polar plane can survive a long time in this configuration.
