The Settling of Warped Disks in Oblate Dark Halos

Abstract

When a galaxy forms, the disk may initially be tilted with respect to a flattened dark halo. The misalignment between the disk and the halo is a common explanation for galactic disk warps, since in this state, disks have precessing bending modes which resemble real warps. A dubious assumption in this theory is that the gravitational response of the halo is negligible. We therefore calculate the response of an oblate halo to a precessing inclined exponential disk using a variety of techniques. We construct models with a rigid exponential disk precessing in a particle halo, a particle disk precessing inside a static bulge/halo potential, and a self-consistent model with a particle disk, bulge and halo. When the disk:halo mass ratio is small ($\sim$10\%) within 5 exponential scale radii, the disk settles to the equatorial plane of the halo within 5 orbital times. When the disk and halo mass are comparable, the halo rapidly aligns with the disk within a few orbital times, while the disk inclination drops. The rapid response of the halo to a inclined disk suggests that the warps seen in galactic disks are not likely the bending modes of a precessing disk inside of triaxial halos. If a galaxy forms inclined to the principal plane of a dark halo, either the disk will settle to a principal plane or the inner halo will twist to align with the disk. The outer halo will remain misaligned for a much longer time and therefore may still exert a torque. Warped bending modes may still exist if the misalignment of the outer halo persists for a Hubble time.