Quantifying the Milky Way, LMC and their interaction using all-sky kinematics of outer halo stars

Abstract: The recent pericentric passage of the Large Magellanic Cloud (LMC) through the Milky Way (MW) has dislodged its centre of mass, inducing a state of dynamical disequilibrium, the reflex motion, in the kinematics of outer stellar halo stars. Using data out to distances of $160 \, \rm kpc$ from the combined H3+SEGUE+MagE outer halo survey, we constrain the mass of the MW and LMC, as well as the resulting reflex motion and the velocity anisotropy of the stellar halo. Using a suite of 32,000 rigid MW--LMC simulations, each with a MW stellar halo evolved to the present day in the combined MW--LMC potential, we perform Simulation Based Inference by training a neural posterior estimator on the means and dispersions of the radial and tangential velocities of stars from the combined H3+SEGUE+MagE outer halo sample. Relative to halo stars at $100 \, \rm kpc$, we find the magnitude of the reflex velocity to be $v_{\rm travel} = 39.4^{+7.6}_{-7.2}\,\rm km \, s^{-1}$. Simultaneously, we determine the enclosed MW mass to be $M_{\rm MW}(< 50 \, \rm kpc) = 3.63 \pm 0.16 \times 10^{11}\, \rm M_{\odot}$ and the enclosed LMC mass to be $M_{\rm LMC}(< 50 \, \rm kpc) = 9.74^{+2.07}_{-1.81} \times 10^{10}\, \rm M_{\odot}$. Our results suggest that the total LMC mass must be at least $\sim20\%$ that of the MW. The velocity anisotropy prior to the LMC's infall is constrained to be $β_0 = 0.61 \pm 0.03$. Finally, we demonstrate that failing to account for the LMC in models biases the MW mass estimate to prefer slightly more massive values.