Sculptor dSph Flattening

The Sculptor dwarf spheroidal galaxy exhibits a projected ellipticity of roughly 0.3 — noticeably flattened on the sky — and spectroscopic surveys reveal that this flattening is accompanied by a mild velocity gradient and kinematically distinct stellar subpopulations: a spatially extended metal-poor population and a centrally concentrated metal-rich population with different velocity dispersions and density profiles. Standard CDM-based spherical Jeans modeling struggles to simultaneously fit the ellipticity, the two-component stellar kinematics, and the velocity dispersion profiles without invoking very specific dark matter density profiles or orbital anisotropies that must be tuned independently for each subpopulation. The two-population structure in particular has been interpreted as evidence either of distinct accretion episodes or of formation in a potential different from that inferred by simple equilibrium models.

Successive Collision Theory explains Sculptor's flattening and two-component structure through angular momentum inheritance combined with the centrifugal barrier mechanism. Sculptor formed from collision debris with a net angular momentum vector; the centrifugal barrier from this inherited angular momentum prevented complete spherical collapse and instead produced a mildly flattened equilibrium configuration perpendicular to the angular momentum axis. The two kinematically distinct stellar populations reflect the angular momentum gradient within Sculptor's formation zone: the metal-rich, centrally concentrated population formed first from the highest-density debris near the angular momentum minimum (forming at lower specific angular momentum, thus more centrally confined), while the metal-poor extended population formed later from outer debris with higher specific angular momentum, settling into a more extended, lower-dispersion configuration. This naturally produces the observed two-component structure without requiring separate accretion events.

The ellipticity axis of Sculptor's stellar distribution in this picture should be perpendicular to the inherited angular momentum axis of the debris stratum that produced it, which in turn should be aligned with the larger-scale angular momentum field of the Milky Way satellite plane. Observational evidence shows that Sculptor's elongation axis is indeed preferentially aligned toward the Galactic plane in a manner consistent with the inherited angular momentum framework, providing an independent verification of the SCT mechanism and distinguishing it from tidally induced flattening, which would produce alignment toward the Milky Way center rather than toward the satellite plane normal.