Ultraviolet spectra of extreme nearby star-forming regions ---
  approaching a local reference sample for JWST
  Nearby dwarf galaxies provide a unique laboratory in which to test stellar
population models below $Z_\odot/2$. Such tests are particularly important for
interpreting the surprising high-ionization UV line emission detected at $z>6$
in recent years. We present HST/COS ultraviolet spectra of ten nearby
metal-poor star-forming galaxies selected to show He II emission in SDSS
optical spectra. The targets span nearly a dex in gas-phase oxygen abundance
($7.8<12+\log\mathrm{O/H}<8.5$) and present uniformly large specific star
formation rates (sSFR $\sim 10^2$ $\mathrm{Gyr}^{-1}$). The UV spectra confirm
that metal-poor stellar populations can power extreme nebular emission in
high-ionization UV lines, reaching C III] equivalent widths comparable to those
seen in systems at $z\sim 6-7$. Our data reveal a marked transition in UV
spectral properties with decreasing metallicity, with systems below
$12+\log\mathrm{O/H}\lesssim 8.0$ ($Z/Z_\odot \lesssim 1/5$) presenting minimal
stellar wind features and prominent nebular emission in He II and C IV. This is
consistent with nearly an order of magnitude increase in ionizing photon
production beyond the $\mathrm{He^+}$-ionizing edge relative to H-ionizing flux
as metallicity decreases below a fifth solar, well in excess of standard
stellar population synthesis predictions. Our results suggest that often
neglected sources of energetic radiation such as stripped binary products and
very massive O-stars produce a sharper change in the ionizing spectrum with
decreasing metallicity than expected. Consequently, nebular emission in C IV
and He II powered by these stars may provide useful metallicity constraints in
the reionization era.
