Identify which proper motion measurements for Leo IV and Leo V are closer to the true values

Determine whether the Hubble Space Telescope-derived proper motions or the Gaia DR3 proper motions for the ultra-faint dwarf galaxies Leo IV and Leo V are closer to their true values, in order to resolve which measurements should be used when testing the proposed Crater–Leo group infall scenario and deriving the satellites’ orbital histories.

Background

The paper investigates whether the dwarf galaxies Leo II, Leo IV, Leo V, Crater II, and the globular cluster Crater 1 were accreted into the Milky Way as a group. Robust assessment of group infall requires accurate proper motions to compare predicted and observed phase-space properties. The authors present new HST proper motions for Leo IV and Leo V and compare them against existing Gaia DR3 measurements.

While the new HST measurements improve alignment with the hypothesized great-circle and orbital pole coherence, there remains uncertainty regarding which dataset (HST or Gaia DR3) provides the more accurate proper motions for Leo IV and Leo V. Resolving this is crucial because the choice of proper motion dataset affects inferred orbital properties, group membership, and conclusions about typical group infall events in the Milky Way. Future observational improvements are expected to reduce uncertainties and settle this question.