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

ABSTRACT 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 build-up of the passive galaxy mass functions. We find, for example, that a typical PSB ($M_\ast \sim 10^{10}$ M$_\odot$) at $z\approx 1$ 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 ($M_\ast >10^{10}$ M$\, _\odot$) of the passive mass function at $1 < z < 2$ can be explained by PSBs, with the contribution declining to $\sim 5$ per cent by $z \simeq 0.5$. Accounting for mergers and rejuvenation reduces the inferred PSB contribution by approximately a factor of two. At lower stellar masses ($M_\ast < 10^{10}$ M$_\odot$), rapid quenching through a PSB phase explains a significantly larger fraction of the growth in the passive mass function. With a visibility time of $\sim$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.