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

TL;DR

This study uses a mock lightcone from the SAM L-Galaxies, tuned to the UKIDSS UDS, to quantify how mergers, rejuvenation, and PSB visibility impact the growth of the quiescent galaxy population across $0.5<z<3$ and across stellar masses. By extracting empirical merger and rejuvenation rates directly from the simulated lightcone and applying PSB visibility times that depend on mass, the authors revise the inferred PSB contribution to passive buildup. They find that PSBs account for roughly $18{-}28 ext{\%}$ of the high-mass passive growth at $1<z<2$ (falling to $ ext{≈}5 ext{\%}$ by $z\,\sim0.5$), with mergers and rejuvenation reducing this figure by about a factor of two. At low stellar mass, PSBs explain a substantial (often dominant) fraction of the passive buildup ($60{-}80 ext{\%}$), suggesting a distinct, mass-dependent quenching pathway; overall, the work highlights the critical roles of mergers, rejuvenation, and PSB visibility in modeling the emergence of the red sequence at cosmic noon.

Abstract

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 ($M_\ast\sim10^{10}$\,M$_\odot$) at $z\approx1$ 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 ($M_\ast>10^{10}$\,M$\,_\odot$) of the passive mass function at $1<z<2$ can be explained by PSBs, with the contribution declining to $\sim5$ 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.

Figures (8)

• Figure 1: Galaxies from the Mock lightcone (top panel, generated using L-galaxies), compared to galaxies from the UDS catalogue (lower panel) wilkinson_starburst_2021. Observationally derived stellar masses and redshifts are shown in both panels. Galaxies are color-coded by type: blue for star-forming, red for passive, and orange for post-starburst (PSB). Coloured solid lines indicate the 95% stellar mass completeness limits for each galaxy type as a function of redshift. Notable differences emerge between the Mock and UDS distributions, particularly at higher redshifts. For instance, the Mock contains significantly fewer galaxies with masses above $10^{10.5}$ M$_\odot$ at $z > 2$ compared to the UDS and significantly more low-mass PSB galaxies.
• Figure 2: The SMFs from the Mock (dashed lines) and the UDS (solid lines) across the specified redshift ranges (upper panels). Blue, red, and orange lines represent star-forming, passive, and post-starburst (PSB) galaxies, respectively. Shaded regions indicate the 1$\sigma$ uncertainty on the UDS SMFs. The lower panels display the differences between the SMFs of the Mock and UDS. The SMFs include the orphan correction described in Section \ref{['sec:orphan']}. At lower redshifts, the Mock reproduces the UDS trends well for star-forming and passive galaxies but overpredicts the number of PSBs. At higher redshifts, however, significant deviations between the two datasets become evident.
• Figure 3: The fraction of PSB galaxies that are satellites in the L-galaxies mock dataset, as a function of redshift and stellar mass. The solid black line describes the 95% mass completeness limit for PSB galaxies in the UDS, truncated at $10^{9.5}$ M$_\odot$ to represent the lower limit for resolving galaxies effectively in L-galaxies. The majority of low-mass PSB galaxies are satellites at $z\raisebox{-0.6ex}{$\,\stackrel {\raisebox{-.2ex}{$<$}}{\sim}\,$} 1.5$, with the high-redshift, high-mass population being dominated by central galaxies. Similar evidence for a low-mass satellite population has been found in the observational data (e.g. socolovsky_enhancement_2018, maltby_structure_2018, wilkinson_starburst_2021)
• Figure 4: The measured likelihood that a galaxy will undergo a merger as a function of time since observation in the L-galaxies mock dataset. Low mass galaxies below $10^{10.5}$ M$_\odot$ are shown in the upper row of panels. Galaxies above $10^{10.5}$ M$_\odot$ are shown in the bottom row. The total number of PSB galaxies, quiescent galaxies, and star-forming galaxies with valid merger rates in each bin are shown in each plot as the orange, red, and blue text respectively. The vertical dashed grey line indicates a time of 1.5 Gyrs after observations. This reflects the typical time period between the redshift bins used in this paper. Low-mass passive galaxies (red lines) and low-mass PSBs (orange lines) show significantly higher merger rates compared to a typical SF galaxy and all high-mass galaxy types.
• Figure 5: The measured rejuvenation rates for passive galaxies in the L-galaxies mock dataset, as a function of time since observation, displayed in four redshift bins. All passive galaxies in the indicated mass and redshift bin are examined at later times to see if they would then be classified as star-forming. As rejuvenation requires a galaxy to first be quiescent, only currently passive galaxies are tested. The vertical grey dashed line indicates a fixed time of 1.5 Gyr. The measured rejuvenation rates peak at $\sim 1.5 - 2$ Gyr after detection on the lightcone and then decrease with increasing time since the initial detection of the passive galaxies. This decrease is primarily because a rejuvenated SF galaxy often naturally re-quenches into a passive galaxy at later times, and thus would not be counted as a rejuvenated galaxy despite having passed through a rejuvenation phase.