Abstract

Abstract: The intergalactic medium was rapidly ionized at 6 < z < 9. However, the source of cosmic reionization is hotly debated because there appears to be an insufficient number of high-redshift active galactic nuclei, while neutral gas in star-forming galaxies efficiently absorbs the ionizing photons produced by stars. Impending observatories (e.g., JWST and ELTs) aspire to determine the source of reionization, but the large average opacity of the high-redshift intergalactic medium precludes direct observations of ionizing photons. Fortunately, local analogs can be used to develop indirect diagnostics of the escape of ionizing photons. Here, I present restframe ultraviolet observations of star-forming galaxies that emit ionizing photons at redshifts of 0 and 2. These observations encode details of how ionizing photons escape star-forming galaxies, while also accurately predicting the escape fraction of ionizing photons. These observations provide a template for JWST and ELTs to determine the source of cosmic reionization.