The connection between dusty star-forming galaxies and the first massive quenched galaxies

High-redshift (z ≳ 2) massive quiescent galaxies (MQs) provide an opportunity to probe the key physical processes driving the fuelling and quenching of star formation in the early Universe. Observational evidence suggests a possible evolutionary link between MQs and dusty star-forming galaxies (DSFGs, or sub-millimetre galaxies), another extreme high-redshift population. However, galaxy formation models have historically struggled to reproduce these populations–especially simultaneously–limiting our understanding of their formation and connection, particularly in light of recent JWST findings. In previous work we presented a recalibrated version of the L-Galaxies semi-analytic model that provides an improved match to observationally inferred number densities of both DSFG and MQ populations. For this work we used this new model to investigate the progenitors of MQs at z > 2 and the physical mechanisms that lead to their quenching. We find that most MQs at z > 2 were sub-millimetre-bright (S₈₇₀ ≳ 1 mJy) at some point in their cosmic past. The stellar mass of MQs is strongly correlated with the maximum sub-millimetre flux density attained over their history, and this relation appears to be independent of redshift. However, only a minority of high-redshift DSFGs evolve into MQs by z = 2. The key distinction between typical DSFGs and MQ progenitors lies in their merger histories: MQ progenitors experience an early major merger that triggers a brief, intense starburst and rapid black hole growth, depleting their cold gas reservoirs. In our model, active galactic nucleus (AGN) feedback subsequently prevents further gas cooling, resulting in quenching. In contrast, the broader DSFG population remains sub-millimetre-bright; star formation proceeds primarily via secular processes, and becomes quenched later. These findings provide a coherent theoretical framework for the formation of high-redshift MQs and clarify their connection to DSFGs, highlighting the role of mergers and AGN feedback in shaping the evolution of the most massive galaxies in the early Universe.