  In the last few years, prominent high-ionization nebular emission lines
(i.e., OIII], CIII], CIV, HeII) 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 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 HeII and double-peaked, resonantly-scattered CIV 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 LyA and CIV are needed to understand how ionizing radiation is transmitted
through both low- and high-ionization gas. Future rest-frame FUV observations
of galaxies within the epoch of reionization using the JWST or ELTs will allow
us to constrain the escape of helium-ionizing photons and provide an estimate
for their contribution to the reionization budget.
