Abstract

The Epoch of Reionization (EoR; z ≥ 6) marked the transition of our universe from a neutral to an ionized state, driven by early galaxies and possibly active galactic nuclei (AGN). However, the sources of ionizing Lyman Continuum (LyC) radiation and the mechanisms enabling its escape from largely neutral galaxies remain uncertain. Local (z = 0−0.3) analogs to the EoR galaxies with LyC detections, such as Green Peas (GPs), can illuminate both ionizing sources and LyC escape mechanisms likely at play in the early universe. This thesis establishes radiation-driven feedback, rather than the traditional supernova (SN) feedback, as a major mechanism for promoting LyC escape in GPs and GP-like galaxies. With case studies of resolved iconic starbursts Mrk 71 and Haro 11, as well as an analysis of galaxies from the Low-redshift Lyman Continuum Survey (LzLCS) and literature GPs, I show that this process may dominate in young, high-ionization starbursts with low metallicities. In Haro 11, new HST/COS observations pinpoint the LyC sources. The results underscore the importance of intrinsic LyC luminosity from young stars, rather than simply optical depth, in promoting LyC escape. My findings separately demonstrate the likely importance of anisotropic LyC escape through a dense, cloudy medium. In Mrk 71, I use spatially resolved, Gemini-N/GMOS-IFU observations to show that its broad, 3000 km/s emission-line wings originate from a radiation-driven superwind composed of tiny, dense, neutral gas clumps with a low filling factor, and accelerated by LyC and/or Lyα photons, thus likely implying LyC escape. I therefore propose that similar broad, smooth, power-law wings may be an indicator of LyC escape via such superwinds. I then use Magellan/MIKE, VLT/X-shooter, and WHT/ISIS spectroscopic observations of a sample of 26 LzLCS galaxies, to find that those with higher ionization parameters and lower metallicities have [O III]λ5007 emission-line wings with power-law shapes. The power-law wing parameters correlate with tracers of radiation-dominated conditions, such as O32 and EW(Hβ), and are consistent with the wind kinematics and picket-fence radiative transfer expected under these conditions. Gaussian line wings, in contrast, are associated with higher metallicity, lower ionization, and older ages, and likely trace conventional SN-driven feedback. The above findings thus imply an emerging paradigm of LyC escape defined by two different feedback modes: radiation-dominated and SN-dominated.