Reflex motion in the Milky Way stellar halo resulting from the Large Magellanic Cloud infall

Abstract

ABSTRACT We present the results of N-body models of the Milky Way and Large Magellanic Cloud system and study the kinematic reflex motion in the stellar halo owing to the barycentre displacement of the disc. In agreement with previous studies, we find that the Milky Way disc may be moving at 40 $\, {\rm km~s^{-1}}$ relative to the barycentre prior to the Large Magellanic Cloud infall. The resulting reflex motion is visible in tangential velocities of the stellar halo as a simple dipole. The signal is strongest for stars with long dynamical times, identifiable in position-velocity data as stars with large apocentres, whose dynamical memory is still well represented by the unperturbed Milky Way potential. The signal varies across the sky depending on the stellar tracer and may be up to the same magnitude as the velocity of the disc centre of mass, making reflex motion a source of bias for Milky Way potential determinations based on the modelling of stellar streams and/or smooth halo tracers such as blue horizontal branch or RR Lyrae stars.