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

Understand

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 $z>6$ ($\mathrm{W_{C IV, 0}} \gtrsim 20$ {\AA}) are uniformly stronger than observed locally ($\mathrm{W_{C IV, 0}} \lesssim 2$ {\AA}), 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.