Ultraviolet ( UV ) observations of local star-forming galaxies have begun to establish an empirical baseline for interpreting the rest-UV spectra of reionization-era galaxies .
However , existing high-ionization emission line measurements at z > 6 ( W _ { C { \sc IV } , 0 } { } \gtrsim 20 Å ) are uniformly stronger than observed locally ( W _ { C { \sc IV } , 0 } { } \lesssim 2 Å ) , likely due to the relatively high metallicities ( Z / Z _ { \odot } > 0.1 ) typically probed by UV surveys of nearby galaxies .
We present new HST /COS spectra of six nearby ( z < 0.01 ) extremely metal-poor galaxies ( XMPs , Z / Z _ { \odot } \lesssim 0.1 ) targeted to address this limitation and provide constraints on the highly-uncertain ionizing spectra powered by low-metallicity massive stars .
Our data reveal a range of spectral features , including one of the most prominent nebular C iv doublets yet observed in local star-forming systems and strong He ii emission .
Using all published UV observations of local XMPs to-date , we find that nebular C iv emission is ubiquitous in very high specific star formation rate systems at low metallicity , but still find equivalent widths smaller than those measured in individual lensed systems at z > 6 .
Our moderate-resolution HST /COS data allow us to conduct an analysis of the stellar winds in a local nebular C iv emitter , which suggests that some of the tension with z > 6 data may be due to existing local samples not yet probing sufficiently high \mathrm { \alpha / Fe } abundance ratios .
Our results indicate that C iv emission can play a crucial role in the JWST and ELT era by acting as an accessible signpost of very low metallicity ( Z / Z _ { \odot } < 0.1 ) massive stars in assembling reionization-era systems .