All-sky Kinematics of the Distant Halo: The Reflex Response to the LMC

The infall of the Large Magellanic Cloud (LMC) is predicted to displace the inner Milky Way (MW), imprinting an apparent “reflex motion” on the observed velocities of distant halo stars. We construct the largest all-sky spectroscopic data set of luminous red giant stars from 50-160 kpc, including a new survey of the southern celestial hemisphere. We fit the full 6D kinematics of our data to measure the amplitude and direction of the inner MW’s motion toward the outer halo. The observed velocity grows with distance such that, relative to halo stars at 100 kpc, the inner MW is lurching at ≈40 km s⁻¹ toward a recent location along the LMC’s past orbit. Our measurements align with N-body simulations of the halo’s response to a 1.8 × 10¹¹ M_⊙ LMC on first infall, suggesting that the LMC is at least 15% as massive as the MW. Our findings highlight the dramatic disequilibrium of the MW outskirts, and will enable more accurate measurements of the total mass of our Galaxy.