10.25394/PGS.8029226.v1 Ke Shi Ke Shi Understanding the early stage of cluster formation Purdue University Graduate School 2019 galaxies: clusters: general galaxies: evolution galaxies:formation galaxies: high-redshift cosmology:observations Astrophysics Cosmology and Extragalactic Astronomy 2019-06-11 14:45:52 Thesis https://hammer.purdue.edu/articles/thesis/Understanding_the_early_stage_of_cluster_formation/8029226 Understanding the formation and evolution of galaxies is a crucially important task in modern astronomy. It is well known that galaxy formation is strongly affected by the environments they reside in. Galaxy clusters, as the densest large-scale structures in the Universe, thus serve as ideal laboratories to study how galaxy formation proceeds in dense environments. Clusters already began to form at $z>2$, therefore to directly witness the early stage of galaxy formation in dense environments, it is necessary to identify progenitors of clusters (`protoclusters') and study their galaxy constituents within. In this thesis, I present two observational studies on high-redshift protoclusters at $z>3$. Utilizing multiwavelength data and different galaxy selection techniques, significant galaxy overdensities are found in the two protoclusters, which are predicted to evolve into Coma-like clusters by present day. Various types of galaxies are identified in the protocluster, such as normal star-forming galaxies, massive quiescent galaxies and post-starburst galaxies. Together with extreme and rare sources such as giant Lyman-alpha nebulae and brighest cluster galaxy, they paint a picture of how different galaxy populations trace the underlying dark matter halos. Finally, the environmental impact on galaxy properties appears to be a subtle one for these protoclusters, which might depend on the galaxy population one chooses to study.