Is the Milky Way ringing? The hunt for high-velocity streams

Abstract

We perform numerical simulations of a stellar galactic disc with initial conditions chosen to represent an unrelaxed population which might have been left following a merger. Stars are unevenly distributed in radial action angle, though the disc is axisymmetric. The velocity distribution in the simulated solar neighbourhood exhibits waves travelling in the direction of positive ν, where u, ν are the radial and tangential velocity components. As the system relaxes and structure wraps in phase space, the features seen in the u-ν plane move closer together. We show that these results can be obtained also by a semi-analytical method. We propose that this model could provide an explanation for the high-velocity streams seen in the solar neighbourhood at approximate ν in kms-1, of -60 (HR1614), -80, -100 (Arcturus) and -160. In addition, we predict four new features at ν ≈ -140, -120, 40 and 60kms-1. By matching the number and positions of the observed streams, we estimate that the Milky Way disc was strongly perturbed ~1.9Gyr ago. This event could have been associated with Galactic bar formation.