Determine the Orbital Stage of Interacting Galaxy Pairs

Determine whether an individual galaxy pair is approaching or moving away from close passage using projected separation and morphological disturbance, despite projection effects and the non-monotonic evolution of pair separation.

Background

The paper interprets strongly disturbed pairs at small projected separations as systems near close passage and strongly disturbed pairs at intermediate separations as possible post-passage systems whose current star formation has begun to decline while longer-timescale stellar-population signatures remain enhanced. However, projected separation does not evolve monotonically during a merger, and tidal disturbances can persist after pericentric passage. Consequently, the same projected separation and disturbance level can correspond to different stages of the interaction.

The unresolved issue is whether observations of projected separation and morphological disturbance alone can identify the direction of orbital evolution for an individual pair—specifically, whether it is moving toward or away from close passage. Resolving this would require additional orbital information or independent stage indicators beyond the observables analyzed here.