Direct Constraints on the Extremely Metal-poor Massive Stars Underlying Nebular C iv Emission from Ultra-deep HST/COS Ultraviolet Spectroscopy

Peter Senchyna, Daniel P. Stark, Stéphane Charlot, Adele Plat, Jacopo Chevallard, Zuyi Chen, Tucker Jones, Ryan L. Sanders, Gwen C. Rudie, Thomas J. Cooper, Gustavo Bruzual

Research output: Contribution to journalArticlepeer-review

11 Scopus citations


Metal-poor nearby galaxies hosting massive stars have a fundamental role to play in our understanding of both high-redshift galaxies and low-metallicity stellar populations. But while much attention has been focused on their bright nebular gas emission, the massive stars that power it remain challenging to constrain. Here we present exceptionally deep Hubble Space Telescope ultraviolet spectra targeting six local (z < 0.02) galaxies that power strong nebular C iv emission approaching that encountered at z > 6. We find that the strength and spectral profile of the nebular C iv in these new spectra follow a sequence evocative of resonant scattering models, indicating that the hot circumgalactic medium likely plays a key role in regulating C iv escape locally. We constrain the metallicity of the massive stars in each galaxy by fitting the forest of photospheric absorption lines, reporting measurements driven by iron that lie uniformly below 10% solar. Comparison with the gas-phase oxygen abundances reveals evidence for enhancement in O/Fe 2-4 times above solar across the sample, robust to assumptions about the absolute gas-phase metallicity scale. This supports the idea that these local systems are more chemically similar to their primordial high-redshift counterparts than to the bulk of nearby galaxies. Finally, we find significant tension between the strong stellar wind profiles observed and our population synthesis models constrained by the photospheric forest in our highest-quality spectra. This reinforces the need for caution in interpreting wind lines in isolation at high redshift, but also suggests a unique path toward validating fundamental massive star physics at extremely low metallicity with integrated ultraviolet spectra.

Original languageEnglish
Article number105
JournalAstrophysical Journal
Issue number2
StatePublished - May 1 2022

Bibliographical note

Funding Information:
The authors thank the anonymous referee for very helpful comments which improved the clarity of this manuscript. This research is based on observations made with the NASA/ESA Hubble Space Telescope and supported by a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programs GO:15646 and GO:15881. Observations reported here were obtained at the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution. Some of the computing for this project was performed on the Memex cluster. We would like to thank Carnegie Institution for Science and the Carnegie Sci-Comp Committee for providing computational resources and support that contributed to these research results.

Funding Information:
P.S. was generously supported by a Carnegie Fellowship through the Carnegie Observatories during the completion of this manuscript. D.P.S. acknowledges support from the National Science Foundation through the grant AST-1410155. J.C. and S.C. acknowledge support from the ERC via an Advanced Grant under grant agreement No. 321323-NEOGAL.

Publisher Copyright:
© 2022. The Author(s). Published by the American Astronomical Society.

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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