The persistence of warps in spiral galaxies with massive haloes

We study the persistence of warps in galactic discs in the presence of massive haloes. A disc is approximated by a set of massive rings, while a halo is represented by a conventional n-body simulation. We confirm the conclusion of Nelson & Tremaine that a halo responds strongly to an embedded precessing disc. This response invalidates the approximations made in the derivation of classical 'modified tilt' modes. We show that the response of the halo causes the line of nodes of a disc that starts from a modified tilt mode to wind up within a few dynamical times. We explain this finding in terms of the probable spectrum of true normal modes of a combined disc-halo system. The key physical point is that in each radial range the halo rapidly aligns with the disc, so calculations based on the assumption that, in the presence of a warped disc, the halo retains a regular spheroidal structure are based on a fatally flawed assumption.