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ABSTRACT We investigate the evolutionary histories of a population of high-mass, high-redshift, quiescent galaxies in the cosmohydrodynamical simulation Thesan, studying the characteristic properties of their haloes and environments over the epoch of reionization. Thesan employs a modified version of the Arepo moving-mesh code utilized in IllustrisTNG, which incorporates on-the-fly radiative transfer to couple haloes and galaxies with the evolving radiation field. Thesan exhibits nine massive quiescent galaxies at z=5. 5, in a (95. 5 cMpc) ³ volume, with no counterpart in IllustrisTNG. A numerical issue in the simulation reduces active galactic nucleus (AGN) feedback efficiency by a factor of 25 while enhancing accretion rates, creating a regime of suppressed feedback. We find their stellar mass assembles rapidly through smooth halo accretion in dense environments, particularly from massive neighbouring structures, while their early-forming haloes develop fast-growing potential wells hosting massive black holes. This suppressed feedback allows prolonged black hole growth before eventual kinetic-mode quenching, providing insight into galaxy evolution under weakened AGN regulation. We find that megaparsec-scale overdensities and halo masses continue growing after quenching, suggesting these galaxies will reside in some of the largest haloes and densest regions of space by z=6. With massive quiescent galaxies found in James Webb Space Telescope data, the identification of such galaxies in Thesan enables isolation of halo and environmental conditions most conducive to their evolution under this suppressed feedback regime, guiding future deep surveys and N-body simulation studies of analogous systems.
Chittenden et al. (2026) studied this question.