A Case-Study of Grand-Design Warps in Galactic Disks

We present the results of a case-study of a small number of typical galaxies with integral-shaped warps: applying a new method to directly fit tilted-ring models to high-quality HI data cubes obtained with the WSRT and the VLA, we confirm and extend the "Rules for the behaviour of Warps" established by Briggs in 1990. Up until now these put the strongest constraints on theories of ex-tragalactic, symmetric warps and their formation. Broadly in accordance with other studies of warped galaxies, we can state the following features of our tilted-ring parametrisations, tentat-ively formulated as new rules for the behaviour of (symmetric, in-tegral-shaped) warps: 1) At large radii, the orientation of the disk becomes constant again. The galaxy therefore shows a two-disk structure, consisting of an in-ner disk being aligned with the stellar body, and a more or less flat outer disk, the warp being the transition from one orientation to an-other. 2) Under the assumption of circular orbits, a change of the rotation velo-city occurs at the transition radius, where the warp starts. 3) The HI surface density drops at the transition radius and the slope of the surface-density profile becomes shallow beyond the transition ra-dius. These results suggest the existence of a common bending mode of the disk-Dark Matter halo system in which the outer, flattened halo pre-cesses uniformly about the inner, precessing portion of the halo that is aligned with the stellar disk. Within the errors, however, they are partly also com-patible with recent warp formation scenarios, which connect warping to secondary cosmic infall.