Jet Stream Forecast to Encounter Twice as Many Dark Matter Subhalos as Any Other Milky Way Stream

The study derives initial conditions for each stream to match present-day observations then tracks tidal stripping and subhalo flybys over cosmic time. Subhalos that reach the inner galaxy are stripped to 1-20% of peak mass and prove twice as compact as earlier models assumed, raising the expected perturbation signal. Jet, AAU, GD-1 and Palomar 5 head the ranking; the top ten streams together record 2-12 encounters each. A spectroscopic campaign reaching sub-km s^{-1} precision could therefore map the low-mass subhalo population below the galaxy-formation threshold.

Connecting these forecasts to actual dark-matter models remains limited by uncertainty in the abundance and radial distribution of subhalos that survive to the solar neighborhood. The galactic disk's influence on stripping is included, yet baryonic feedback prescriptions and the precise inner slope of the subhalo mass function still vary across simulations. Future work that folds in Gaia DR4 proper motions and multi-epoch radial-velocity data will tighten the initial conditions and allow direct comparison of observed gap statistics with the predicted encounter rates.

Observational programs already targeting GD-1 and Pal 5 can be expanded to Jet and AAU with modest additional telescope time. If velocity substructure at the 0.1 km s^{-1} level is detected in the next five years, the result would directly constrain warm-dark-matter or self-interacting-dark-matter parameter space that is otherwise inaccessible to direct-detection experiments.