Physical driver of the outer-halo contraction

Determine the physical mechanism responsible for the inward contracting motion of the Milky Way outer stellar halo inferred from DESI DR2 K-giant radial velocities, specifically the distance-independent negative compression term (mu_compr ≈ −18 km s−1) obtained in the dipole-plus-contraction velocity field model over 50–120 kpc, and disentangle it from the Large Magellanic Cloud–induced reflex dipole motion.

Background

The authors model the outer-halo radial-velocity field (50–120 kpc) with a dipole-plus-contraction parameterization and find both a reflex dipole consistent with the Large Magellanic Cloud (LMC) infall and a global inward contraction characterized by a negative compression term (mu_compr).

This contracting motion helps explain why the southern halo shows stronger negative radial velocities than the north, yet the underlying cause of the contraction is not identified. The amplitude appears roughly constant with distance, while the dipole term grows with radius and the dipole apex shifts with distance.

Although simulations and various hypotheses (e.g., LMC effects, unrelaxed substructures) are mentioned, the specific physical driver of the contraction remains unidentified, prompting a focused determination of its origin beyond the measured phenomenology.