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

Abstract

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 $z>6$ (${\mathrm{W}}_{\mathrm{CIV},0}\gtrsim 20$ Å) are uniformly stronger than observed locally (${\mathrm{W}}_{\mathrm{CIV},0}\lesssim 2$ 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 $\alpha /Fe$ 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.

17 pages, 9 figures, published in MNRAS