Lin, Yu-Heng2023-11-282023-11-282023-06https://hdl.handle.net/11299/258793University of Minnesota Ph.D. dissertation. June 2023. Major: Physics. Advisor: Claudia Scarlata. 1 computer file (PDF); xviii, 168 pages.Cosmic reionization is the last major global transition in which intergalactic neutral hydrogen is mainly ionized by active star-forming galaxies. However, it is unclear whether bright or faint star-forming galaxies play the major role in reionization. Since direct observation of high-redshift galaxies is infeasible, detailed understanding of these galaxies relies on studying their low-redshift counterparts, which are low-mass, low-metallicity galaxies identified by their extreme emission lines.The goal of this thesis is to study star-forming galaxies in the early universe. I first study low-metallicity galaxies identified in the Dark Energy Survey. These galaxies have extreme emission lines and are the local counterparts of high-redshift galaxies. Inferring from the studies of low redshift galaxies, I construct an empirical model that estimates the contribution of high-redshift galaxies to reionization. Obscured active galactic nuclei have emission line spectra similar to star-forming galaxies. I introduce a high-redshift obscured AGN and how to distinguish active galactic nuclei and star-forming galaxies using rest-frame UV emission lines. I also present two cooperated works on star-forming galaxies in high redshift. I show how these galaxies can provide information about the neutral fraction during the epoch of reionization, and how they are similar to the extreme emission line galaxies found in low redshift. Finally, to determine the contribution of faint galaxies to the reionization, we propose to measure their Lyman continuum escape fraction by observing the Lyman-alpha profile of their low redshift counterpart.enactive galaxiesdwarf galaxiesreionizationThesis or Dissertation