Warp Excitation in the Milky Way Galaxy by the Magellanic Clouds

Abstract

The question of the origin of warps in spiral galaxies is an open one. Edge-on bending features are observed in nearly every spiral galaxy, including our own, meaning that any proposed excitation mechanism must be nearly ubiquitous as well. One such mechanism is excitation by a satellite perturber, in which a satellite galaxy makes repeated orbits around a larger host galaxy, inducing warps via halo wake amplification. The Milky Way is thought to form such a system with the Magellanic Clouds; indeed, their orbits are nearly optimal for warp production. However, recent observational results suggest that the Magellanic Clouds are only now on their first passage about the Milky Way, in which case it is not obvious that warping of the observed amplitude could have been induced. To investigate this, we simulate two Milky Way-Magellanic Cloud systems over 10 Gyr, one with high halo mass (2 x 10^12 M.; MC orbital period 4 Gyr) and one with low halo mass (1 x 10^12 M.; MC orbital period 10 Gyr). We find that the latter, a first-infall model, actually produces more warping than the former, a repeated-orbit scenario. We suggest an explanation based on disk heating, which can suppress warping, since the repeated orbits excite more vertical velocity dispersion at radii where the warps occur. While neither model produces warps more than one-half of the observed amplitudes, we have demonstrated the non-obvious result that warping can indeed occur in first-infall models, and that these warps can even exceed those of multiple-passage scenarios.