Unveiling the population of massive quenched galaxies at z ≥2 in the COLIBRE simulations – I. Galaxy demographics

The James Webb Space Telescope has uncovered a substantial population of Massive ((Formula presented) ), Quenched Galaxies (MQGs) in the early Universe ((Formula presented) ), whose properties challenge current galaxy formation models. In this series, we examine this population of MQGs within the new colibre cosmological hydrodynamical simulations, which introduce key innovations in their sub-grid physics. In this first paper, we find a dependence of MQG number densities on both mass resolution and the Active Galactic Nucleus feedback implementation, as well as a significant impact from potential observational uncertainties. Using the fiducial (Formula presented) volume L200m6 simulation, which provides adequate volume, mass, and spatial resolution to study these systems, we report number densities and stellar mass functions in broad agreement with the latest observations. The predicted quenching and formation time-scales are qualitatively consistent with observational inferences, indicating extended formation (medians (Formula presented) ) followed by rapid quenching (medians (Formula presented) ) with strong starburst episodes. Leveraging the state-of-the-art physics in colibre, the model predicts that MQGs have dust and (Formula presented) fractions more than 1 dex lower than their massive star-forming counterparts; generally consistent with the (scarce) observational estimates. MQGs and massive star-forming systems show broadly similar stellar sizes and kinematics, suggesting that size or morphological transformations occur after quenching in colibre. Our results provide robust predictions for MQGs and show that tensions with observations are reduced when an effective observational uncertainty is forward-modelled.