LMC Disk Integrity

The Large Magellanic Cloud maintains a well-defined stellar disk with coherent rotation, a bar, and an active star-forming interstellar medium despite its current position deep within the Milky Way's halo at roughly 50 kpc. In ΛCDM the LMC is typically modeled as undergoing its first close approach to the Milky Way, a scenario motivated by HST proper motion measurements suggesting a high orbital velocity that implies the LMC may be on a nearly parabolic rather than a bound orbit. This first-infall interpretation is necessary to explain why the LMC retains its disk at all: a system on multiple close passages would have been tidally heated and its disk disrupted long ago. However, the first-infall hypothesis creates its own tensions — it implies the LMC has not been significantly pre-processed by the Milky Way's environment, yet the Magellanic Stream stretching 150 degrees across the sky requires a long history of gas stripping that is difficult to produce in a single pericentric passage.

Successive Collision Theory resolves this tension through the stabilizing effect of angular momentum inheritance on disk structure. The LMC formed from collision debris endowed with a high specific angular momentum fraction of the total collision J, corresponding to a debris stratum well away from the collision axis. This high inherited angular momentum set a strong centrifugal barrier that made the LMC's disk intrinsically more resistant to tidal disruption than a CDM subhalo of equivalent mass would be. The centrifugal barrier continuously acts to re-circularize stellar orbits perturbed by Milky Way tides, preserving the disk plane against disruption even during close passages. The LMC therefore does not need to be on its first infall to retain its disk; it retains its disk because the inherited angular momentum budget is sufficient to maintain disk integrity through multiple pericenters in a way that CDM subhalos — which owe their kinematics entirely to violent relaxation rather than primordial angular momentum inheritance — do not.

The Magellanic Stream in this picture formed over multiple orbital periods through gradual ram-pressure stripping and tidal extraction of gas from the LMC's outer disk, consistent with its long angular extent and the age distribution of its stellar content. The LMC's disk integrity and the Stream's length are therefore not in contradiction but jointly explained: the disk is stabilized by angular momentum inheritance while the Stream records the cumulative gas extraction history of a galaxy on a long-standing orbit within the Milky Way's extended halo environment. SCT predicts that the LMC's orbital history will, when fully reconstructed with future proper motion precision, reveal multiple pericentric passages rather than a single first approach.