The Epoch of Reionization or Cosmic Dawn is the period of time between the Dark Ages, when the universe was mainly comprised of neutral hydrogen and there were few sources of light, and around z∼6 (or∼1 billion years after the big bang) by which time the universe’s hydrogen had been completely reionized. This period in time in which the first sources of light emerged was one of the most significant changes that the universe experienced: not only for the intergalactic medium, but also for galaxy and structure formation. Despite its importance and impact on the history of the universe, today, the timing and process of reionization remain largely unconstrained. There are several barriers to placing constraints on the Epoch of Reionization. One of the largest ones is that neutral hydrogen precludes the observation of the most accessible emission line, Lyα, making it difficult to observe past z∼6. In addition, the faintest galaxies, which are likely the main drivers of reionization, are far beyond the detection limit of current telescopes in blank fields.Gravitational lensing has a long history of aiding in the discovery of high-redshift galaxies by magnifying them to a brighter apparent magnitude, making them observable with limited telescope time. In this dissertation, I use gravitational lensing to probe some of the faintest galaxies ever observed during the Epoch of Reionization. I begin in Chapter 2 by detailing a lens model of a well-studied galaxy cluster, Abell 370. In order to model its mass and magnification distribution with the ultimate goal of correcting high-redshift galaxies residing behind the cluster for magnification, I useHubble Space Telescope imaging of singly and multiply imaged galaxies as constraints, and a free-form lens modeling code called Strong and Weak Lensing United. I compare my results to other lens models of the cluster and discuss the systematic differences. This work is an important contribution to the high redshift galaxy community, as it adds a lens model which uniquely does not assume that light traces mass, and solves for a best fit model non-parametrically.In Chapters 3 and 4, I focus on the high-redshift galaxy population behind a different setof galaxy clusters: the 41 clusters from the Reionization Lensing Cluster Survey (RELICS).xiiiIn Chapter 3, I discuss the highest-redshift candidates from the Lyman-break selected sample, all at or above z∼8. In Chapter 4, I detail the entire z≥5.5 sample. An important component in both chapters is the use of imaging from theSpitzer Space Telescope to con-strain the rest-frame optical spectrum of the galaxy candidates. I go into depth using two methods of spectral energy distribution fitting and discuss systematic uncertainties. In both chapters, I discuss several exciting candidates for follow-up with current and future tele-scopes. These include a spatially resolved z∼10 arc, a galaxy with a likely evolved stellar population at z∼8, and a galaxy likely containing extreme [OIII]+Hβemission.In Chapter 5, I discuss the observation and discovery of an extreme Lyman-α emitting galaxy at z∼7, also from the RELICS sample, named the Dichromatic Primeval Galaxy at z∼7 (DP7). In this Chapter, I raise several questions about DP7’s physical properties and highlight it as an exciting galaxy for followup with JWST and ALMA. Along with strong(>200 ̊A rest-frame equivalent width) and spatially-resolved Lyman-α, DP7 shows signs of a red UVβslope (∼−1) and possibly multiple components in the UV.In Chapter 6, I summarize: with imaging fromHubble Space Telescope and Spitzer SpaceTelescope and spectroscopy from the Keck Telescopes to study lensed galaxies at high-redshift, I have been able to address questions about the Epoch of Reionization through observations of galaxies, focusing mainly on constraining properties of galaxies at the EoR, with an eye towards constraining their effect on the intergalactic medium with future tele-scopes.
