Exploring the Production of High-Energy Photons with Reionization-Era Analogue Galaxies

The early universe underwent a spectacular phase change as the gas between galaxies was rapidly ionized. Currently, the observational and theoretical toolbox cannot discern between accretion onto black holes or hot massive stars as the source of ionizing photons for this cosmic reionization. The key to understanding the physical processes responsible for reionizing the universe are the nearby galaxies that are analogues to galaxies during the Epoch of Reionization. Recently, an exciting population of extremely low-metallicity star-forming galaxies have been discovered in the local universe. This sample provides the first laboratories of massive star populations at metallicities below 10% solar, similar to the galaxies during the Reionization Era. Using deep FUV and optical spectra of two of these extremely low-metallicity galaxies, we have found that both galaxies have unprecedentedly large He II, Ar IV, and C IV extremely high-ionization optical and FUV emission lines. We demonstrate that canonical photoionization models, using typical stellar population models, catastrophically fail to reproduce the high-ionization emission lines. We propose an additional ionizing source (X-ray binaries or shocks) in the models is necessary to reproduce the entire suite of emission lines observed in extreme emission-line galaxies. To test this scenario, we analyze Chandra observations of these two galaxies to constrain the high-energy sources.