Publication Date:
2019-07-11
Description:
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 ($
m W_{C, {scriptscriptstyle IV},0}{} gtrsim 20$ Å) are uniformly stronger than observed locally ($
m W_{C, {scriptscriptstyle 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$odot$ ≲ 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 α/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.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
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