Abstract

The neutral gas between galaxies was rapidly reionized at redshifts between 6-10. The origin of these ionizing photons remains elusive, but fundamentally shapes the subsequent evolution of the galaxies and the formation of large-scale structure in the universe. The key to unraveling the epoch of reionization is understanding both the sources emitting ionizing photons (stars vs. active galactic nuclei) and how these ionizing photons escape galaxies.
Unfortunately, we will not be able to directly observe the ionizing continuum at high redshifts because the intergalactic medium is neutral. Thus, we need indirect methods to determine the sources of ionizing photons. I will focus on the Mg II emission line as a new probe of the escape of ionizing photons. Spatial maps of Mg II emission from local emitters of ionizing photons demonstrates both how and how many ionizing photons can escape from galaxies. This sets the stage for upcoming telescopes to use Mg II to determine how the early universe became reionized.