Building blocks of the universe: The search for galaxy protoclusters

Galaxy clusters are the most massive virialized structures in the presentday Universe, containing thousands of galaxies. Since the dawn of the concept of galaxies, astronomers have been studying the distribution of galaxies across the sky and found them to be nonsymmetrical, with certain parts having high concentrations of galaxies in close proximity. To understand the formation and evolution of galaxy clusters, it is necessary to investigate their progenitors, the socalled
protoclusters that inhabit the early Universe, since information pertaining to their evolution is lost as part of the virilization process. To study protoclusters, they must first be located, which is done through a number of different methods
utilizing multiwavelength data from the ultraviolet to the radio parts of the electromagnetic spectrum. Observers face challenges when it comes to locating protoclusters, as sufficient data over a large area is needed to distinguish the overdense protocluster from the field environment. Spectroscopic followup is often required to determine the systemic redshift of the protocluster, as many protoclusters are initially identified as overdensities of photometrically selected galaxies. In this thesis, I go through the history and development of galaxy cluster studies and their progenitors, galaxy protoclusters. I explain the observational and theoretical advancements that have led to our current understanding
of protoclusters. Next, I present my first paper which goes into how to locate and study protocluster candidates using largescale photometric survey data in the COSMOS field. This work contributes to our current understanding of protoclusters by comparing them with our currently accepted models for protocluster evolution. The candidates can be used by the community as prime targets for followup studies by current and future instruments. My second paper presents a spectroscopic followup of one of the protocluster candidates I located in the COSMOS field. By utilizing the DEIMOS instrument on the Keck telescope, I search for detections of the Lymanα emission line, which can be used to determine spectroscopic redshifts to the protocluster candidate galaxies. This contributes to the small sample of high redshift protocluster that has currently been observed. Lastly, I present my work on a compact galaxy group, which has detections of multiple lines in both mission and absorption and the continuum emission from ALMA.