Cluster Candidates with Massive Quiescent Galaxies at z ∼ 2

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 greater than or similar to 2. In this paper, we report a discovery of seven cluster candidates with massive quiescent galaxies at z similar to 2 in the 3.5 deg(2) 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 similar to 26, K-s similar to 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 (log(M & lowast;/M circle dot)>10.5) and quiescent (log(sSFR[yr-1])<-10) galaxies at z similar to 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) x 10(44) 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<log(sSFR[yr-1])<-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 similar to 2.