Field Halo Streams (GD-1, Pal 5)

The GD-1 and Palomar 5 stellar streams are among the longest and most kinematically cold tidal streams known in the Milky Way's halo, stretching across tens of degrees on the sky with velocity dispersions of only a few km/s. Their extreme coldness makes them extraordinarily sensitive probes of the gravitational potential along their tracks, and detailed modeling of their gaps, wiggles, and density variations has been used to argue for perturbations from dark matter subhalo flybys at the rate predicted by CDM. However, the detailed morphology of both streams — particularly the characteristic off-track features and gap distributions — does not match pure CDM subhalo impact statistics cleanly, and GD-1's "spur" feature and density variations appear more consistent with a single coherent perturbation than a stochastic rain of subhalo encounters, creating tension with the smooth CDM substructure prediction.

In Successive Collision Theory, cold stellar streams trace the inherited angular momentum field of the Milky Way's halo directly. The parent comoving frame hierarchy establishes a large-scale tidal field with specific spatial structure derived from the collision geometry; as GD-1's progenitor cluster was tidally disrupted, its stars spread along their orbital path within this structured tidal environment. The gaps and density variations in GD-1 and Pal 5 in SCT are produced primarily by the large-scale gravitational superposition of the nested frame hierarchy — a smooth but spatially varying potential that creates density enhancements and gaps at locations corresponding to resonances between the orbital frequency of stream stars and the characteristic frequencies of the parent frame tidal field — rather than by stochastic dark matter subhalo impacts. This produces a characteristic gap distribution that is more regular and correlated than pure CDM subhalo statistics predict.

The spur feature in GD-1 is naturally explained in this context as a density enhancement formed where stream stars passing through a resonance zone of the parent frame tidal field are momentarily compressed and then diverge from the main track. Because the parent frame tidal field is coherent rather than stochastic, SCT predicts that gaps and spurs in long cold streams will exhibit a quasi-periodic spacing set by the orbital resonances rather than the random spacing produced by subhalo impacts — a prediction testable with complete radial velocity surveys of the stream populations that upcoming spectroscopic facilities will enable.