M31 Satellite Plane

Andromeda (M31) hosts a Great Plane of Andromeda comprising roughly half of its known dwarf satellite galaxies arranged in a thin, co-rotating disk-like structure roughly 400 kpc in diameter with a thickness of only ~14 kpc. Line-of-sight velocity measurements confirm that the satellites on one side of M31 are predominantly receding while those on the other side are approaching, demonstrating genuine co-rotation rather than a chance spatial alignment. The probability of this configuration arising from isotropic random infall in ΛCDM is estimated at less than 0.1 percent in cosmological simulations, making it one of the most severe small-scale structure challenges alongside the Milky Way's own satellite plane. The fact that both the Milky Way and M31 — the two dominant members of the Local Group — independently host flattened co-rotating satellite planes is a particularly acute problem for any purely stochastic assembly scenario.

In Successive Collision Theory, both the Milky Way and M31 condensed from the same collision debris field and therefore both inherited angular momentum from the same parent collision event. The angular momentum vector J of the initial pocket collision imprinted a preferred orbital plane throughout the debris field over the entire volume that eventually became the Local Group. M31 and the Milky Way formed at different positions within this debris volume but drew their angular momentum from the same parent distribution; their satellite systems formed from local sub-fields of this same angular momentum reservoir. The result is that satellite planes around both galaxies are aligned with the same Large Scale angular momentum stratum, explaining not only the existence of each plane individually but also the observed tendency for the two planes to share a common orientation relative to the Local Group geometry.

The coherent co-rotation of the M31 satellite plane — rather than a random mix of prograde and retrograde orbits — follows directly from the fact that angular momentum inheritance is a coherent vector operation, not a random assignment. Each satellite accretes with angular momentum aligned to its local patch of the debris field, and because that patch is a smooth derivative of the global J vector, neighboring satellites end up co-rotating. This is a generic prediction of SCT for any galaxy that formed within the debris of a single collision event, and the simultaneous confirmation of co-rotating planes around both major Local Group spirals constitutes strong observational support for the inherited angular momentum framework.