Intense C iv and He ii Emission in z ∼ 0 Galaxies: Probing High-energy Ionizing Photons

In the last few years, prominent high-ionization nebular emission lines (i.e., O iii], C iii], C iv, and He ii) have been observed in the deep UV spectra of z ∼ 5–7 galaxies, indicating that extreme radiation fields characterize reionization-era systems. These lines have been linked to the leakage of Lyman continuum photons (necessary for reionization) both theoretically and observationally. Consequently, high-ionization UV emission lines present our best probe to detect and characterize the most distant galaxies that we will observe in the coming years, and are key to understanding the sources of reionization, yet the physics governing their production is poorly understood. Here we present recent high-resolution Hubble Space Telescope spectra of two nearby extreme UV emission-line galaxies, J104457 and J141851. We report the first observations of intense nebular He ii and double-peaked, resonantly scattered C iv emission, a combination that suggests these galaxies both produce and transmit a significant number of very high-energy ionizing photons (E > 47.89 eV) through relatively low column densities of high-ionization gas. This suggests that, in addition to photons at the H-ionizing edge, the very hard ionizing photons that escape from these galaxies may provide a secondary source of ionization that is currently unconstrained observationally. Simultaneous radiative transfer models of Lyα and C iv are needed to understand how ionizing radiation is transmitted through both low- and high-ionization gas. Future rest-frame far-UV observations of galaxies within the epoch of reionization using the James Webb Space Telescope or extremely large telescopes (ELTs) will allow us to constrain the escape of helium-ionizing photons and provide an estimate for their contribution to the reionization budget.