The role of mergers and rejuvenation in the buildup of the quiescent population at cosmic noon

We investigate the quenching of galaxies using a mock observational lightcone generated from the Semi-Analytic Model (SAM) L-Galaxies, closely matched to observations from the UKIDSS Ultra Deep Survey (UDS). The sample is used to study merging, rejuvenation, and visibility times for star-forming, quiescent, and post-starburst (PSB) galaxies, to assess the impact on the build-up of the passive galaxy mass functions. We find, for example, that a typical PSB (í µí± * ∼ 10 10 M ⊙) at í µí± § ≈ 1 has a 15 per cent likelihood of merging and around a 25 per cent likelihood of rejuvenating within 1 Gyr of being identified. Applying these rates and timescales to the observational data, we estimate the fraction of quiescent galaxies that passed through a PSB phase. We find that 18 − 28 per cent of the build-up in the massive end (í µí± * > 10 10 M ⊙) of the passive mass function at 1 < í µí± § < 2 can be explained by PSBs, with the contribution declining to ∼ 5 per cent by í µí± § ≃ 0.5. Accounting for mergers and rejuvenation reduces the inferred PSB contribution by approximately a factor of two. At lower stellar masses (í µí± * < 10 10 M ⊙), rapid quenching through a PSB phase explains a significantly larger fraction of the growth in the passive mass function. With a visibility time of ∼ 0.75 Gyr, we find that around 60 − 80 per cent of low-mass passive galaxies underwent a PSB phase. Our findings provide further evidence that low-and high-mass galaxies follow different quenching pathways.