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Deep Extragalactic Neutral Hydrogen Surveys with the Green Bank Telescope Nickolas Pingel The Green Bank Telescope (GBT) is utilized to map the galaxies of the Westerbork Radio Synthesis Telescope (WSRT) Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) Survey to look for diffuse Hi features that were previously undetected due, in part, to the lack of angular sensitivity of the WSRT. Cosmological simulations predict that cold gas should accrete onto galaxies in the nearby Universe in the form of vast filaments that penetrated into the halo and extend down to the disk. While such filaments are too hot to be observed in Hi emission towards the edge of the virial radius, it is thought that the gas will cool near enough disk to be observed in Hi near the disk at Hi column densities on the order of 1018 cm−2. We find no evidence for significant large scale Hi features that could be associated with the theorized cold flows in the HALOGAS sample of galaxies an Hi column density level of 1018 cm−2. This result suggests future Hi surveys should invest the necessary time to probe down to Hi column densities ∼1017 cm−2 in order to detect Hi associated with infalling cold flows and the cosmic web. In addition to going down to rarely probed Hi column density levels, this work also presents the results of commissioning observations of a cryogenically cooled phased array feed for the GBT that is optimized for observations between one and two GHz. Maps of an external galaxy and faint Hi cloud compare well to data already obtained with the traditional single pixel feed, indicating this instrument will be a definite to the current single pixel feed horn. The ability of PAFs to simultaneously form multiple beams on the sky increases the field of view, and thus survey speeds, such that Hi column density levels below 1018 cm−2 can be reached in a realistic time constraints. Observing the Universe at these column density levels will reveal galaxy evolution mechanisms that so far have remained undetected. Acknowledgements First and foremost, I wish to thank my Mom and Dad, Robin and Michael Pingel. Growing up as an only child meant that, for better or worse at times, their attention to my development was undivided. From a young age I was taught to approach every aspect of life — whether that be a sport, academics, or general interest — full heartedly. This dissertation is the ultimate collimation of that philosophy. I love and appreciate you both in ways that cannot be expressed in the few words alloted here. Thank you for your unwavering support. To Carolyn, the love of my life, your support and love has and always will abide. We took a chance entering the adventure that was graduate school at West Virgina University and moving to Morgantown. We’ve cultivated an incredibly community both here and back in Madison. And while it will be difficult to leave them both temporarily, I am excited to build new communities with you wherever we go. I wish to be the same source of encouragement for the pursuit of your passions as you have provided me over the years. I can’t wait to travel the world with you. I of course also wish to thank my adviser, Dr. D. J. Pisano. Allowing me to begin research my first year of graduate school helped immensely in jump-starting my development as a scientist. Whenever I came to you with something to chat about — whether that be a concept I did not initially understand or an exciting result — you always made the time necessary time to guide me to the right interpretation on my own. I cannot stress enough how integral you were in helping me achieve my dream of becoming a professional astronomer. I look forward to working together for many years to come. In terms of other influential advisers, I wish to also thank Dr. Snezana Stanimirović, who took a risk on sophomore at the University of Wisconsin-Madison with no astronomy background whatsoever. I am immensely proud of the work that we accomplished over the subsequent four years, and I am happy that it continued even into my time at WVU. Thank you for taking a chance on me. To my friends that I grew up with, I realize it is particularly rare to have a friend group that stays close even as we spread out throughout the U.S. and even to Australia. I love you all. To my friends I made in Morgantown — and my band mates in [Indistinct Chattering] — you all helped to keep me sane and grounded while I was finishing up this Dissertation. I’ll never forget playing at 123 Pleasant Street or just the nights watching movies and goofing around. And I need to give one final thanks to my friends at Chestnut Brew Works, who gave me the fuel I needed to finish this process with a smile of my face.