The Debris of the "Last Major Merger" is Dynamically Young

Understand

The Milky Way's (MW) inner stellar halo contains an [Fe/H]-rich component with highly eccentric orbits, often referred to as the "last major merger." Hypotheses for the origin of this component include Gaia-Sausage/Enceladus (GSE), where the progenitor collided with the MW proto-disk 8-11 Gyr ago, and the Virgo Radial Merger (VRM), where the progenitor collided with the MW disk within the last 3 Gyr.

• These two scenarios make different predictions about observable structure in local phase space, because the morphology of debris depends on how long it has had to phase mix.
• The recently-identified phase-space folds in Gaia DR3 have positive caustic velocities, making them fundamentally different than the phase-mixed chevrons found in simulations at late times.
• Roughly 20\% of the stars in the prograde local stellar halo are associated with the observed caustics.