Galactic Warps Induced By Cosmic Infall

Abstract

Recent ideas for the origin and persistence of the warps commonly observed in disc galaxies have focused on cosmic infall. We present N-body simulations of an idealized form of cosmic infall onto a disc galaxy and obtain a warp that closely resembles those observed. The inner disc tilts remarkably rigidly, indicating strong cohesion due to self-gravity. The line of nodes of the warp inside $R_{26.5} ~ 4.5 \Rd$ is straight, while that beyond $R_{26.5}$ generally forms a loosely-wound, leading spiral in agreement with Briggs's rules. We focus on the mechanism of the warp and show that the leading spiral arises from the torques from the misaligned inner disc and its associated inner oblate halo. The fact that the line of nodes of most warps forms a leading spiral might imply that the disc mass is significant in the centre. If the line of nodes can be traced to very large radii in future observations, it may reveal information on the mass distribution of the outer halo. The warp is not strongly damped by the halo because the precession rate of the inner disc is slow and the inner halo generally remains aligned with the inner disc. Thus even after the imposed quadrupolar perturbation is removed, the warp persists for a few Gyrs, by which time another infall event can be expected.