2026ApJ...997...90W

Velocity dispersion (σ) in stellar streams from globular clusters (GCs) is sensitive to heating by Galactic substructure, including dark matter (DM) subhalos. Recent studies have compared σ in observed and modeled streams to probe DM properties, but have relied on stream models that neglect strong encounters, black holes (BHs), and mass segregation in GCs. Such phenomena may inflate stream σ or introduce selection effects-e.g., a σ that depends on star mass (m). We investigate this prospect using Monte Carlo N-body simulations of GCs under static Galactic tides to generate mock streams with realistic mass and velocity distributions. We find σ correlates with m, especially after core collapse (the GC's observable increase in central density upon ejecting its BHs), rising from 1.2 to 2.2 km s^–1 between m = 0.3-0.8 M_⊙, with typical kinematic cuts on stream membership. Similar in magnitude to heating by Galactic substructure, this enhancement occurs because the GC's loss of BHs allows its most-massive stars to occupy its dense center, raising their likelihood of strong ejection via binary interactions and adding broad, exponential wings to the stream's velocity distribution. Streams' kinematics thus probe properties (density, BH retention) of their progenitor GCs. Our results also imply observations of streams from some GCs, especially those not subject to highly episodic mass loss, may select for higher σ than predicted by models neglecting σ's m dependence. This would cause observed σ in streams-already on the low side of expectations for cold DM-to further favor alternatives such as warm or ultralight DM.