Milky Way Halo Vibrations and Incommensurate Stream Velocities
Collisionless dark matter galactic halos are expected to exhibit damped oscillations as a result of ongoing late time accretion. An n-body model of the cosmological assembly of a Milky-Way-like halo is used to quantify the time dependence of its gravitational field. The simulation contains stellar streams, with velocities transverse to the path of the stream comparable to those reported for the Orphan stream. The fluctuations in the quadrupole moment of the dark matter halo are measured to be about 1% of the radial component of the gravity field, but are sufficient to largely explain the perpendicular velocities. The largest late time sub-halo within 100 kpc is a 2% mass at about 70 kpc from the galactic center. Simulations of self-interacting dark matter halos have reduced potential fluctuations as the collisions create a viscosity. If velocity measurements of a larger sample of Milky Way streams finds (or does not find) the expected distribution of transverse velocities, it will lead to limits on the cross section of self-interacting dark matter.