Self‐consistent response of a galactic disc to vertical perturbations

We study the self‐consistent, linear response of a galactic disc to vertical perturbations, as induced, say, by a tidal interaction. We calculate the self‐gravitational potential corresponding to a non‐axisymmetric, self‐consistent density response of the disc using the Green's function method. The response potential is shown to oppose the perturbation potential because the self‐gravity of the disc resists the imposed potential, and this resistance is stronger in the inner parts of a galactic disc. For the m = 1 azimuthal wavenumber, the disc response opposes the imposed perturbation up to a radius that spans a range of 4–6 disc scalelengths, so that the disc shows a net warp only beyond this region. This physically explains the well known but so far unexplained observation that warps typically set in beyond this range of radii. We show that the inclusion of a dark matter halo in the calculation only marginally changes (by ∼10 per cent) the radius for the onset of warps. For perturbations with higher azimuthal wavenumbers, the net signature of the vertical perturbations can only be seen at larger radii – for example, beyond 7 exponential disc scalelengths for m = 10 . Also, for the high‐ m cases, the magnitude of the negative disc response due to the disc self‐gravity is much smaller. This is shown to result in corrugations of the mid‐plane density, which explains the puzzling scalloping with m = 10 detected in H  i in the outermost regions ∼30 kpc in the Galaxy.