Ursa Minor Structure

The Ursa Minor dwarf spheroidal galaxy displays several structural features that are difficult to reconcile with simple equilibrium models in a standard CDM potential. Its stellar body is elongated with an ellipticity of roughly 0.56 — among the most flattened of the classical dSphs — and detailed mapping has revealed a secondary density clump or substructure offset from the main body by roughly 0.5 kpc in projection. The presence of this off-center substructure, sometimes called the "cold clump," implies either that Ursa Minor has experienced a recent interaction that disturbed its stellar distribution, or that it possesses an unusually shallow and extended dark matter potential that cannot fully mix stellar substructure on a dynamical timescale. In a cuspy CDM halo with the short central dynamical time that the cusp implies, the cold clump should have phase-mixed away within a few Gyr; its persistence therefore requires a central density that is far lower than CDM predicts, adding to the core-cusp tension at the scale of this specific system.

Successive Collision Theory explains both Ursa Minor's extreme flattening and its persistent cold substructure through the interplay of angular momentum inheritance and the flat central density core that angular momentum produces. Ursa Minor condensed from a collision debris stratum with a relatively high specific angular momentum, producing a strongly flattened equilibrium shape perpendicular to the inherited angular momentum axis — consistent with the observed ellipticity of 0.56. The flat central density core that results from the centrifugal barrier means that the dynamical friction timescale for any substructure in the central region is very long: without a steep central density gradient to drive rapid phase mixing, kinematic substructures can persist for many Gyr after their formation or after a minor perturbation such as a close tidal encounter with a passing field object. The cold clump in Ursa Minor is a substructure in this long phase-mixing limit, not evidence of an ongoing interaction.

The extreme flattening of Ursa Minor relative to other classical dSphs reflects its particular position in the collision debris angular momentum distribution: it formed from a stratum with angular momentum oriented favorably relative to the observer's line of sight, producing a large apparent ellipticity, and with a magnitude that exceeds the typical debris element's angular momentum, producing a deeper centrifugal barrier and thus more pronounced flattening. SCT predicts a continuous distribution of dSph ellipticities set by the angular momentum distribution of the collision debris, with the most elongated systems forming from the highest-angular-momentum strata — a prediction consistent with the observed range of dSph shapes across the Local Group population and the tendency for more flattened dSphs to also show larger apparent rotation signatures from velocity gradient measurements.