AAS 205th Meeting, 9-13 January 2005
Session 122 HI and Galactic Disks
Oral, Wednesday, January 12, 2005, 10:00-11:30am, Pacific Salon 1

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[122.04] Galactic Warps Formed through Cosmic Infall

J. Shen, J. A. Sellwood (Rutgers University)

The extended HI disks of many edge-on spiral galaxies appear noticeably warped away from the inner disk with an ``integral sign'' shape. At least half, perhaps all, of spiral galaxies are warped. The origin and maintenance of warps are still not well understood. We use fully self-consistent N-body simulations to study the effect of cosmic infall on an isolated disk galaxy, which we find to be a promising way of making warps. The amplitude and morphology of warps formed in an idealized experiment to test this scenario resemble observations closely. The agreement with Briggs (1990)'s rules is also very encouraging: the inner disk tilts remarkably rigidly, indicating the strong cohesion due to the self-gravity; the line of nodes (LON) inside R26.5 ~4.5 Rd is straight; and the LON beyond R26.5 always forms a loosely-wound leading spiral. We show that the leading spiral arises from the torque from the misaligned inner disk. In this scenario the damping of a warp by the halo is weak, because the free precession rate of the inner disk is slow and the inner halo generally remains aligned with the inner disk. Thus warps formed this way can persist for a relatively long time (a few Gyrs), by which time another infall event can be expected. We also point out the spirality of the LON of warps in this idealized model should twist from leading to trailing at very large radii, such feature may be observable in future HI surveys.

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