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.
