On the evolution of galaxy protoclusters from the epoch of reionization to cosmic noon
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Over the last 7–8 billion years, mature galaxy clusters, characterized by massive quiescent galaxy populations residing in > 10¹⁴ M [subscript ☉] dark matter halos, have become ubiquitous features of the cosmic web. It is theorized that these clusters are preceded by a proto-cluster stage, which form at much earlier times in filamentary dark matter overdensities and later collapse into a central dark matter halo. Identifying and characterizing protoclusters between the epoch of reionization and cosmic noon, during which time they are expected to undergo periods of rapid star formation and evolution prior to gravitational collapse, remains in its nascent stages. With a heterogeneous mix of selection techniques, spectroscopic completeness, area coverage, and wavelength coverage, we are still vitally lacking a statistically complete catalog of protoclusters — and even more so a general understanding of their physical properties. This doctoral thesis addresses these questions twofold by: 1) using z ∼ 6 quasars as tracers of overdensities at multiple wavelengths, and 2) presenting a comprehensive X-ray to radio case study of a massive protocluster at z = 2.5. The first half of this thesis focuses on understanding the environments of the brightest quasars at z > 6, under the hypothesis that quasars hosting > 10⁹ M [subscript ☉] black holes signal regions of accelerated growth and mass buildup in the early Universe. In two separate studies, I present the results of searches for enhanced star formation in the vicinities of quasars, using two complementary tracers of star–forming galaxies. Using data from the Atacama Large Millimeter Array, I show that a search for extreme dust–obscured galaxies via millimeter continuum emission yields inconclusive results concerning our ability to detect a true overdensity signal. Using the Hubble Space Telescope to search for the more abundant and less massive population of Lyman break galaxies, I show a heterogeneity of overdensity signals near these quasars. The second half of this thesis focuses on multiwavelength observations of known protoclusters at lower redshifts, where more observational tools are available to constrain their environments. I present an analysis of the member galaxies within a structurally complex protocluster core at z = 2.5 located in the COSMOS extragalactic field, using data from the Atacama Large Millimeter Array, the Very Large Array, and the COSMOS archive. I evaluate possible evolutionary hypotheses on the basis of the morphology of the cold gas needed for future star formation, the spatial distribution of its member galaxies, and the existence of a marginal X-ray detection that all present a challenge to our cosmological understanding of virial collapse of clusters during this epoch. Finally, I present a future outlook on further case studies and large surveys which will be made possible by next generation facilities.