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 light-cone 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 buildup of the passive galaxy mass functions. We find, for example, that a typical PSB (M_∗ ∼ 10¹⁰ M_☉) at z ≈ 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 time-scales 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 (M_∗ > 10¹⁰ M_☉) of the passive mass function at 1 < z < 2 can be
explained by PSBs, with the contribution declining to ∼ 5 per cent by z ≃ 0.5. Accounting for mergers and rejuvenation reduces
the inferred PSB contribution by approximately a factor of two. At lower stellar masses (M_∗ < 10¹⁰ 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.