Formation and Evolution of
Galaxy Clusters Across Cosmic Time

Galaxies are distributed inhomogeneously on small scales in the Universe and thus define large-scale structures. Understanding the evolution of cluster galaxies is to determine precisely how galaxies change their properties as a result of the hierarchical growth of large-scale structures. The difficulty is the need for accurate determination of the redshift to determine the precise environment of the galaxies. We present a novel and unique method to estimate accurate redshifts of star-forming galaxies by measuring the flux ratio of the same emission line observed through two adjacent narrow-band filters. We apply this method to our NB921 and new NB926 data taken with Hyper Suprime-Cam on the Subaru Telescope. We obtain redshifts for [OII] emission line galaxies at z ~ 1.5 and Halpha emitters at z ~ 0.4. We find that the projected celestial distribution does not precisely trace the real distribution of galaxies, indicating the importance of the three-dimensional view of structures to properly identify and quantify galaxy environments. We then investigate the environmental dependence of galaxy properties with local density and some well-defined populations of galaxies. At low redshift, “Red Sequence clusters” are older, more developed and there are cluster-specific quenching processes such as ram pressure stripping, starvation etc which reduce the SF activity. On the other hand, at high redshift, clusters are too young and have not evolved well yet. Galaxies have experienced a similar, steady history of SF. We also find that “Close companion galaxies” are inducing star formation activity. Moreover, there is a significant positive correlation between SFR and overdensity at z ~ 1.5 and 0.4 which is weak or absent locally. These trends are associated with the existence of a population of bright, massive blue galaxies in dense regions.