Detection of the Milky Way reflex motion due to the Large Magellanic Cloud infall

TL;DR

This paper provides evidence that the Milky Way's disc is moving due to the gravitational influence of the Large Magellanic Cloud, affecting stellar kinematics and requiring revised dynamical models.

Contribution

It presents the first detection of the Milky Way's reflex motion caused by the LMC infall using stellar halo data and proper motions.

Findings

Milky Way disc moves at ~32 km/s relative to outer halo stars.

Reflex motion points towards the LMC's past trajectory.

Dynamical models must include LMC-induced perturbations.

Abstract

The Large Magellanic Cloud is the most massive satellite galaxy of the Milky Way, with an estimated mass exceeding a tenth of the mass of the Milky Way. Just past its closest approach of about 50 kpc, and flying by the Milky Way at an astonishing speed of 327 km/s, the Large Magellanic Cloud can affect our Galaxy in a number of ways, including dislodging the Milky Way disc from the Galactic centre-of-mass. Here, we report evidence that the Milky Way disc is moving with respect to stellar tracers in the outer halo ($40<r<120$ kpc) at $v_{\rm travel}=32^{+4}_{-4}$ km/s, in the direction $(\ell,b)_{\rm apex}=(56^{+9}_{-9},-34^{+10}_{-9})$ degrees, which points at an earlier location on the LMC trajectory. The resulting reflex motion is detected in the kinematics of outer halo stars and Milky Way satellite galaxies with accurate distances, proper motions and line-of-sight velocities. Our results indicate that dynamical models of our Galaxy cannot neglect gravitational perturbations induced by the Large Magellanic Cloud infall, nor can observations of the stellar halo be treated in a reference frame that does not correct for disc reflex motion. Future spectroscopic surveys of the stellar halo combined with Gaia astrometry will allow for sophisticated modelling of the Large Magellanic Cloud trajectory across the Milky Way, constraining the dark matter distribution in both galaxies with unprecedented detail.