Carina Velocity Gradient

The Carina dwarf spheroidal galaxy exhibits a significant line-of-sight velocity gradient across its stellar body — a systematic change in mean stellar velocity as a function of position projected on the sky — that exceeds what can be accounted for by simple solid-body rotation of a tidally undisturbed pressure-supported system. In ΛCDM, such velocity gradients in dSphs are interpreted either as evidence of tidal distortion by the Milky Way or as internal rotation inherited from an earlier disk-like precursor phase, yet neither explanation is fully satisfactory: pure tidal models require orbital configurations that are not naturally produced in CDM subhalo merger trees, and the rotation amplitudes exceed what simple collapse from a CDM halo predicts for a system of Carina's mass. The tension is compounded by similar velocity gradients found in other dwarf spheroidals, suggesting a systematic rather than accidental feature.

Successive Collision Theory predicts that every gravitationally bound structure, including dwarf spheroidals, retains a residual angular momentum signature inherited from the collision debris field. Carina formed from debris that carried a specific local angular momentum vector; as it collapsed and virialized, it retained the net rotation corresponding to that inherited angular momentum projected along the observer's line of sight. The amplitude and direction of the velocity gradient encode the local angular momentum density of the debris stratum from which Carina formed, and the gradient's orientation relative to the Milky Way reflects the geometry of the collision axis. The gradient does not require ongoing tidal distortion; it is a primordial kinematic signature that was established at formation and subsequently preserved because two-body relaxation timescales in systems of Carina's stellar mass exceed a Hubble time.

The SCT framework makes a specific testable prediction: the rotation axis implied by the velocity gradient should be statistically aligned with the orbital pole of Carina's orbit around the Milky Way, because both are determined by the same inherited angular momentum vector. Proper motion measurements that constrain Carina's space velocity can test this alignment, and the emerging picture from Gaia data is broadly consistent with such a correlation across the classical dwarf spheroidal population — a pattern that ΛCDM random-accretion scenarios do not predict and cannot easily reproduce.