Star Formation Evolution in Galaxy Pairs: Constraints from Morphological Disturbances and Recent Star Formation Histories

Abstract: Galaxy interactions can enhance star formation, but how star formation evolves during galaxy interactions remains poorly constrained by observations. We combine pair separation, morphological disturbance, and recent star formation history to study this evolution. We measure morphological disturbance with the shape asymmetry parameter ($A_{\rm shape}$) using deep images from the DESI Legacy Surveys, and use Galaxy Zoo DESI classifications as an independent test. We infer recent star formation histories by comparing SFR and $\mathrm{EW}(\mathrm{H}α)$, which trace current star formation, with $\mathrm{EW}(\mathrm{H}δ<em>A)$ and $D_n4000$, which are sensitive to stellar populations formed over longer timescales. At small projected separations, strongly disturbed galaxies show the strongest enhancements in SFR and $\mathrm{EW}(\mathrm{H}α)$, indicating that close encounters trigger strong star formation. At intermediate separations ($d</em>{\rm p}\sim100\,\mathrm{kpc}$), their current star formation is only moderately enhanced, but their $\mathrm{EW}(\mathrm{H}δ_A)$ enhancement is the strongest. This indicates a larger contribution from intermediate-age stars formed during stronger star formation in the past $\sim0.1$--$1\,\mathrm{Gyr}$. The TNG100 analysis shows that this pattern results from rapid changes in SFR around close passage. SFR rises sharply during the encounter and declines as the galaxies move apart, while tidal disturbances remain visible. As short-lived massive stars disappear and intermediate-age A-type stars begin to dominate the spectrum, the $\mathrm{EW}(\mathrm{H}δ_A)$ enhancement peaks later than the SFR enhancement. Our results provide important observational evidence for the evolution of SFR along the merger sequence.