The Clustering of Orbital Poles Induced by the LMC: Hints for the Origin of Planes of Satellites

A significant fraction of Milky Way (MW) satellites exhibit phase-space properties consistent with a coherent orbital plane. Using tailored N-body simulations of a spherical MW halo that recently captured a massive (1.8 1011 M o˙) LMC-like satellite, we identify the physical mechanisms that may enhance the clustering of orbital poles of objects orbiting the MW. The LMC deviates the orbital poles of MW dark matter particles from the present-day random distribution. Instead, the orbital poles of particles beyond R ≈ 50 kpc cluster near the present-day orbital pole of the LMC along a sinusoidal pattern across the sky. The density of orbital poles is enhanced near the LMC by a factor δ ρmax = 30% (50%) with respect to underdense regions and δ ρ iso = 15% (30%) relative to the isolated MW simulation (no LMC) between 50 and 150 kpc (150-300 kpc). The clustering appears after the LMC's pericenter (≈50 Myr ago, 49 kpc) and lasts for at least 1 Gyr. Clustering occurs because of three effects: (1) the LMC shifts the velocity and position of the central density of the MW's halo and disk; (2) the dark matter dynamical friction wake and collective response induced by the LMC change the kinematics of particles; (3) observations of particles selected within spatial planes suffer from a bias, such that measuring orbital poles in a great circle in the sky enhances the probability of their orbital poles being clustered. This scenario should be ubiquitous in hosts that recently captured a massive satellite (at least ≈1:10 mass ratio), causing the clustering of orbital poles of halo tracers.We are grateful for the comments provided by Jenna Samuel, Alex Riley, Isabel Santos-Santos, and Veronica Arias. N.G.-C. and G.B. are supported by NSF CAREER award AST-1941096, NASA ATP award 17-ATP17- 0006, and HST grant AR 15004. E.P. is supported by the Miller Institute for Basic Research in Science at UC Berkeley. This work was supported in part by World Premier International Research Center Initiative (WPI Initiative), MEXT, Japan. F.A. G. acknowledges financial support from FONDECYT Regular 1211370 and from the Max Planck Society through a Partner Group grant. C.L. acknowledges funding from the European Research Council (ERC) under the European Unions Horizon 2020 research and innovation program (grant agreement No. 852839). K.V.J. was supported by NSF grant AST-1715582