UCLA

One of the foremost goals in the study of cosmological reionization is understanding the nature of the sources of the ionizing photons. The search for leaking ionizing radiation from high-redshift star-forming galaxies has resulted in dozens of promising candidates, yet few confirmed detections. In this thesis, I present results from a survey for z~2.85 ionizing Lyman-Continuum (LyC) emission in the HS1549+1933 field and place constraints on the amount of ionizing radiation escaping from star-forming galaxies. Using a custom narrowband filter (NB3420) tuned to wavelengths just below the Lyman limit at z>=2.82, I probe the LyC spectral region of 49 Lyman break galaxies (LBGs) and 91 Lyman-alpha emitters (LAEs) spectroscopically confirmed at z>=2.82. I also present high-resolution, UVJH follow-up HST observations of 16 z~3 candidate LyC emitters identified with the NB3420 filter. With these follow-up data, I obtain high spatial-resolution photometric redshifts of all subarcsecond components of the high-redshift galaxies in order to eliminate foreground contamination and identify robust candidates for leaking LyC emission. I find only one object with a robust LyC detection that is not due to foreground contamination. A comparison with representative samples of LBGs indicates that the most exceptional aspect of the stellar population fit to this object is its young age (<50 Myr). I obtain a contamination-free estimate for the comoving specific ionizing emissivity at z=2.85, indicating (with large uncertainties) that star-forming galaxies provide roughly the same contribution as QSOs to the ionizing background at this redshift. The results of my thesis work show that foreground contamination prevents ground-based LyC studies from obtaining a full understanding of LyC emission from z~3 star-forming galaxies. Future progress in direct LyC searches is contingent upon the elimination foreground contaminants through high spatial-resolution observations, and upon acquisition of sufficiently deep LyC imaging to probe ionizing radiation in galaxies at the faint end of the luminosity function.