Direct Constraints on the Extremely Metal-Poor Massive Stars Underlying
  Nebular C IV Emission from Ultra-Deep HST/COS Ultraviolet Spectroscopy
  Metal-poor nearby galaxies hosting massive stars have a fundamental role to
play in our understanding of both high-redshift galaxies and low metallicity
stellar populations. But while much attention has been focused on their bright
nebular gas emission, the massive stars that power it remain challenging to
constrain. Here we present exceptionally deep Hubble Space Telescope
ultraviolet spectra targeting six galaxies that power strong nebular C IV
emission approaching that encountered at $z>6$. We find that the strength and
spectral profile of the nebular C IV in these new spectra follow a sequence
evocative of resonant scattering models, indicating that the hot circumgalactic
medium likely plays a key role in regulating C IV escape locally. We constrain
the metallicity of the massive stars in each galaxy by fitting the forest of
photospheric absorption lines, reporting measurements driven by iron that lie
uniformly below 10% solar. Comparison with the gas-phase oxygen abundances
reveals evidence for enhancement in O/Fe above solar across the sample, robust
to assumptions about the absolute gas-phase metallicity scale. This supports
the idea that these local systems are more chemically-similar to their
primordial high-redshift counterparts than to the bulk of nearby galaxies.
Finally, we find significant tension between the strong stellar wind profiles
observed and our population synthesis models constrained by the photospheric
forest in our highest-quality spectra. This reinforces the need for caution in
interpreting wind lines in isolation at high-redshift, but also suggests a
unique path towards validating fundamental massive star physics at extremely
low metallicity with integrated ultraviolet spectra.
