2025ApJ...980..104K

Galaxy clusters are crucial to understanding the role of the environment in galaxy evolution. However, due to their rarity, only a limited number of clusters have been identified at z ≳ 2. In this paper, we report a discovery of seven cluster candidates with massive quiescent galaxies at z ∼ 2 in the 3.5 deg² area of the XMM Large Scale Structure field, roughly doubling the known cluster sample at this frontier redshift if confirmed. We construct a photometric redshift catalog based on deep (i ∼ 26, K_s ∼ 24) multiwavelength photometry from the u* band to the K band gathered from the Hyper Suprime-Cam Subaru Strategic Program and other collaborative/public surveys. We adopt a Gaussian kernel density estimate with two different spatial scales (10″ and 60″) to draw a density map of massive ($\mathrm{log}({M}_{* }/{M}_{\odot })\gt 10.5$) and quiescent ($\mathrm{log}(\mathrm{sSFR}\,[{\mathrm{yr}}^{-1}])\lt -10$) galaxies at z ∼ 2. Then, we identify seven prominent overdensities. These candidates show clear red sequences in color-magnitude diagrams (z - H versus H). Moreover, one of them shows an extended X-ray emission with L_X = (1.46 ± 0.35) × 10⁴⁴ erg s^–1, suggesting its virialized nature. There is no clear evidence of enhancement nor suppression of the star formation rate of the main-sequence galaxies in the clusters. We find that cluster galaxies have a higher fraction of transition population with $-10.5\lt \mathrm{log}(\mathrm{sSFR}\,[{\mathrm{yr}}^{-1}])\lt -10$ (12%) than the field (2%), which implies the ongoing star formation quenching. The quiescent fraction in the cluster candidates also exceeds that in the field. We confirm that the excess of a quiescent fraction is larger for higher-mass galaxies. This is the first statistical evidence for the mass-dependent environmental quenching at work in clusters even at z ∼ 2.