Galaxy evolution in the Post-Merger Regime – I. Most merger-induced in situ stellar mass growth happens post-coalescence

Galaxy mergers can enhance star formation rates throughout the merger sequence, with this effect peaking around the time of coalescence. However, owing to a lack of information about their time of coalescence, post-mergers could only previously be studied as a single, time-averaged population. We use time-scale predictions of post-coalescence galaxies in the UNIONS survey, based on the Multi Model Merger Identifier deep learning framework (MUMMI) that predicts the time elapsed since the last merging event. For the first time, we capture a complete timeline of star formation enhancements due to galaxy mergers by combining these post-merger predictions with data from pre-coalescence galaxy pairs in SDSS. Using a sample of 564 galaxies with M_∗ ≥ 10¹⁰M_☉ at 0.005 < z < 0.3 we demonstrate that: (1) galaxy mergers enhance star formation by, on average, up to a factor of two; (2) this enhancement peaks within 500 Myr of coalescence; (3) enhancements continue for up to 1 Gyr after coalescence; and (4) merger-induced star formation significantly contributes to galaxy mass assembly, with galaxies increasing their final stellar masses by, 10 per cent to 20 per cent per merging event, producing on average log(M_∗/M_☉) = 9.56⁺₋₀⁰₁₉¹³ more mass than non-interacting star-forming galaxies solely due to the excess star formation.