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

TL;DR

This study measures the reflex motion of the Milky Way's inner regions caused by the infall of the Large Magellanic Cloud, using a large all-sky spectroscopic dataset of distant halo stars, revealing the galaxy's disequilibrium and informing mass estimates.

Contribution

It provides the first all-sky kinematic measurement of the MW's reflex motion due to the LMC, confirming the LMC's significant mass and its impact on the MW's outer halo.

Findings

Inner MW is moving at ~40 km/s towards the LMC's past orbit.

The LMC's mass is at least 15% of the MW's mass.

The MW's outskirts are in disequilibrium due to the LMC infall.

Abstract

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 dataset 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 towards the outer halo. The observed velocity grows with distance such that, relative to halo stars at $100$ kpc, the inner MW is lurching at $\approx 40$ km s$^{-1}$ towards a recent location along the LMC's past orbit. Our measurements align with N-body simulations of the halo's response to a $1.8 \times 10^{11} M_\odot$ 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.