Satellite Plane Alignments (MW/LMC)

The Milky Way's classical dwarf satellite galaxies are not distributed isotropically around the Galaxy; they lie in a highly flattened, rotating plane — the Vast Polar Structure — that is inclined nearly perpendicular to the Galactic disk and whose members share a common orbital sense. The Large Magellanic Cloud and its associated satellite system reinforce this planar alignment. ΛCDM N-body simulations robustly predict isotropic or mildly flattened satellite distributions drawn from randomly accreted subhalos, and the probability of achieving the observed Milky Way configuration by chance is well below one percent in any large simulated halo sample. This represents a fundamental structural prediction failure that has deepened as proper motion measurements have confirmed the coherent rotation of the plane.

Successive Collision Theory predicts coherent satellite planes as a direct and inevitable consequence of angular momentum inheritance. The initial pocket collision deposited a total angular momentum vector J into the debris field along a specific axis set by the impact geometry. As the Milky Way assembled from this debris, every structure that formed — including the satellite galaxies — inherited angular momentum fractions derived from the same parent vector. Satellites that condensed from debris occupying a common angular momentum stratum naturally share orbital planes aligned with the collision axis, producing the observed rotating satellite plane without invoking special environmental configurations or late-time group infall. The pole of the Vast Polar Structure is thus the fossil record of the collision geometry preserved in the orbital angular momenta of the lowest-mass structures, which are least perturbed by subsequent internal dynamics and therefore retain the primordial angular momentum signature most cleanly.

The alignment of the LMC's own satellite system within the Milky Way's polar plane follows from the same logic: the LMC and its companions condensed from an adjacent angular momentum stratum of the same debris field and therefore share the same inherited orbital orientation. SCT further predicts that the satellite plane should be statistically aligned with both the Milky Way's disk normal and the local filament axis, since all three features trace the same inherited angular momentum hierarchy — a correlation that large proper motion surveys are now beginning to confirm observationally.