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Ultraviolet (UV) observations of local star-forming galaxies have begun toestablish an empirical baseline for interpreting the rest-UV spectra ofreionization-era galaxies. However, existing high-ionization emission linemeasurements at $z>6$ ($\mathrm{W_{C IV, 0}} \gtrsim 20$ {\AA}) are uniformlystronger than observed locally ($\mathrm{W_{C IV, 0}} \lesssim 2$ {\AA}),likely due to the relatively high metallicities ($Z/Z_\odot > 0.1$) typicallyprobed by UV surveys of nearby galaxies. We present new HST/COS spectra of sixnearby ($z<0.01$) extremely metal-poor galaxies (XMPs, $Z/Z_\odot \lesssim0.1$) targeted to address this limitation and provide constraints on thehighly-uncertain ionizing spectra powered by low-metallicity massive stars. Ourdata reveal a range of spectral features, including one of the most prominentnebular C IV doublets yet observed in local star-forming systems and strong HeII emission. Using all published UV observations of local XMPs to-date, we findthat nebular C IV emission is ubiquitous in very high specific star formationrate systems at low metallicity, but still find equivalent widths smaller thanthose measured in individual lensed systems at $z>6$. Our moderate-resolutionHST/COS data allow us to conduct an analysis of the stellar winds in a localnebular C IV emitter, which suggests that some of the tension with $z>6$ datamay be due to existing local samples not yet probing sufficiently high$\mathrm{\alpha/Fe}$ abundance ratios. Our results indicate that C IV emissioncan play a crucial role in the JWST and ELT era by acting as an accessiblesignpost of very low metallicity ($Z/Z_\odot < 0.1$) massive stars inassembling reionization-era systems.

Extremely metal-poor galaxies with HST/COS: laboratories for models of low-metallicity massive stars and high-redshift galaxies