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) has dislodged the Milky Way's (MW) centre of mass, inducing dynamical disequilibrium, the reflex motion, in the kinematics of outer stellar halo stars. Using data out to $160\mathrm{kpc}$ 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 stellar halo velocity anisotropy. 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 $100\mathrm{kpc}$, we find the magnitude of the reflex velocity to be $v_{\mathrm{travel}}=38.6_{-7.8}^{+8.3}\mathrm{km}{\mathrm{s}}^{-1}$. Simultaneously, we determine the enclosed MW mass, $M_{\mathrm{MW}}(<50\mathrm{kpc})=3.36\pm 0.15\times 10^{11}{\mathrm{M}}_{\odot }$ and the enclosed LMC mass, $M_{\mathrm{LMC}}(<50\mathrm{kpc})=8.76_{-1.77}^{+1.94}\times 10^{10}{\mathrm{M}}_{\odot }$. Our results suggest that the total LMC mass must be at least $\sim 20\%$ that of the MW. The velocity anisotropy prior to the LMC's infall is constrained to be ${\beta }_0=0.68\pm 0.02$. Finally, we demonstrate that neglecting the LMC in models biases the estimated MW mass to prefer more massive values.

MNRAS accepted. 17 pages, 12 figures, 2 tables. arXiv admin note: text overlap with arXiv:2510.04735

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