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 local (z < 0.02) 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 2-4 times 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 toward validating fundamental massive star physics at extremely low metallicity with integrated ultraviolet spectra.