Tidal Imprints Of A Dark Sub-Halo On The Outskirts Of The Milky Way

We present a new analysis of the observed perturbations of the HI disk of the Milky Way to infer the existence of a dark sub-halo that tidally interacted with the Milky Way disk. We examine tidal interactions between perturbing dark sub-halos and the gas disk of the Milky Way using high resolution Smoothed Particle Hydrodynamics (SPH) simulations. We compare our results to the observed HI map of the Milky Way to find that the Fourier amplitudes of the planar disturbances are best-fit by a perturbing dark sub-halo with a mass that is one-hundredth of the Milky Way with a pericentric distance of 5 kpc. This best-fit to the Fourier modes occurs about a dynamical time after pericentric approach, when the perturber is 90 kpc from the galactic center. Our analysis here represents a new method to indirectly characterize dark sub-halos from the tidal gravitational imprints they leave on the gaseous disks of galaxies. We also elucidate a fundamental property of parabolic orbits. We show that under certain conditions, one can break the degeneracy between the mass of the perturber and the pericentric distance in the evaluation of the tidal force -- to directly determine the mass of the dark perturber that produced the observed disturbances.