The Role of Small-scale Environments in the Quenching of Massive Galaxies at 1 < z < 5

Massive quiescent galaxies (QGs) at high redshifts are likely progenitors of massive elliptical galaxies in the local Universe. Recent discoveries of high-redshift QGs in overdensities (galaxy groups and protoclusters) have highlighted the relationship between massive galaxy quenching and their surrounding environment. We spectroscopically confirm a galaxy group associated with a massive QG at z spec = 4.53 from the Lyman break feature using Subaru/FOCAS. This group consists of at least three star-forming galaxies within 150 pkpc of the QG, suggesting a physical association necessary for galaxy quenching. To investigate the role of the surrounding environment, we also perform a statistical analysis to characterize the typical environment of QGs at high redshifts. By selecting QGs using a star formation rate-based selection in the COSMOS field, we find that the quiescent fraction is higher in overdensities than in the field at 1 < z < 5. In particular, the elevated quiescent fraction within small-scale overdensities (∼200 pkpc) demonstrates that environmental quenching, possibly driven by galaxy mergers and interactions, plays a major role in the evolution of massive QGs at high redshifts. By extending the low-redshift mass and environmental quenching model up to z ∼ 5, we confirm similar trends to those inferred from the Sloan Digital Sky Survey red galaxy fraction at z < 1. While environmental dependence weakens at z ≳ 3 for the overall population, a clear correlation persists specifically for more massive galaxies. This mass-dependent environmental effect suggests a downsizing trend, highlighting that the accelerated evolution of massive galaxies in high-density regions is already in place in the early Universe.