2025ApJ...989...38C

Measurements of the GD-1 star stream velocity distribution within ±3° of the centerline find a total line-of-sight velocity spread of 5-6 km s^–1 in the-well measured ϕ₁ = [-30, 0] region. The velocity spread is far above the ∼2-3 km s^–1 of a dissolved globular cluster in a smooth galactic potential. Dynamical heating of the GD-1 star stream is simulated in an evolving model Milky Way potential that includes the subhalos extracted from cosmological cold dark matter (CDM) and warm dark matter (WDM) Milky Way-like halos. The model bridges fully cosmological Milky Way-like halos and late-time static Milky Way potentials, allowing individual streams to be accurately integrated. An evolving CDM subhalo population acting for ∼11 Gyr heats GD-1 to 6.2 ± 1.7 km s^–1. The WDM (7 keV and lighter) models develop a velocity dispersion of 3.9 ± 0.2 km s^–1, only slightly greater than the 3.5 km s^–1 in an evolving smooth halo without subhalos for 11 Gyr. The dynamical age of the best model stream is close to the isochrone age of the stars in the stream. Subhalos with masses in the decade around 10^7.5M_⊙, below the mass range of dwarf galaxies, dominate the dynamical stream heating.