Ultraviolet spectra of extreme nearby star-forming regions - approaching a local reference sample for JWST

Peter Senchyna, Daniel P. Stark, Alba Vidal-García, Jacopo Chevallard, Stéphane Charlot, Ramesh Mainali, Tucker Jones, Aida Wofford, Anna Feltre, Julia Gutkin

Research output: Contribution to journalArticlepeer-review

87 Scopus citations


Nearby dwarf galaxies provide a unique laboratory in which to test stellar population models below Z/2. Such tests are particularly important for interpreting the surprising highionization ultraviolet (UV) line emission detected at z > 6 in recent years. We present HST/COS UV spectra of 10 nearby metal-poor star-forming galaxies selected to show He II emission in SDSS optical spectra. The targets span nearly a dex in gas-phase oxygen abundance (7.8 < 12 + logO/H < 8.5) and present uniformly large specific star formation rates (sSFR ~102 Gyr-1). The UV spectra confirm that metal-poor stellar populations can power extreme nebular emission in high-ionization UV lines, reaching CIII] equivalent widths comparable to those seen in systems at z ~ 6-7. Our data reveal a marked transition in UV spectral properties with decreasing metallicity, with systems below 12 + logO/H ≲ 8.0 (Z/Z ≲ 1/5) presenting minimal stellar wind features and prominent nebular emission in He II and C IV. This is consistent with nearly an order of magnitude increase in ionizing photon production beyond the He+-ionizing edge relative to H-ionizing flux as metallicity decreases below a fifth solar, well in excess of standard stellar population synthesis predictions. Our results suggest that often-neglected sources of energetic radiation such as stripped binary products and very massive O-stars produce a sharper change in the ionizing spectrum with decreasing metallicity than expected. Consequently, nebular emission in C IV and He II powered by these stars may provide useful metallicity constraints in the reionization era.

Original languageEnglish (US)
Pages (from-to)2608-2632
Number of pages25
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
StatePublished - Dec 2017
Externally publishedYes


  • Galaxies: evolution
  • Galaxies: stellar content
  • Stars: massive
  • Ultraviolet:galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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