Reconciling the stellar and nebular spectra of high-redshift galaxies

Charles C. Steidel, Allison L. Strom, Max Pettini, Gwen C. Rudie, Naveen A. Reddy, Ryan F. Trainor

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278 Scopus citations


We present a combined analysis of rest-frame far-UV (FUV; 1000-2000 Å) and rest-frame optical (3600-7000 Å) composite spectra formed from very deep Keck/LRIS and Keck/MOSFIRE observations of a sample of 30 star-forming galaxies with z = 2.40±0.11, selected to be broadly representative of the full KBSS-MOSFIRE spectroscopic survey. Since the same massive stars are responsible for the observed FUV continuum and for the excitation of the observed nebular emission, a self-consistent stellar population synthesis model should simultaneously match the details of the FUV stellar+nebular continuum and - when inserted as the excitation source in photoionization models - predict all observed nebular emission line ratios. We find that only models including massive star binaries, having low stellar metallicity (Z/Z≃0.1) but relatively high nebular (ionized gas-phase) abundances (Zneb/Z≃0.5), can successfully match all of the observational constraints. We show that this apparent discrepancy is naturally explained by highly super-solar O/Fe (≃(O/Fe)), expected for a gas whose enrichment is dominated by the products of core-collapse supernovae. While O dominates the physics of the ionized gas (and thus the nebular emission lines), Fe dominates the extreme-UV (EUV) and FUV opacity and controls the mass-loss rate from massive stars, resulting in particularly dramatic effects for massive stars in binary systems. This high nebular excitation - caused by the hard EUV spectra of Fe-poor massive stars - is much more common at high redshift (z ≳ 2) than low redshift due to systematic differences in the star formation history of typical galaxies.

Original languageEnglish (US)
Article number159
JournalAstrophysical Journal
Issue number2
StatePublished - Aug 1 2016


  • H II regions
  • ISM: abundances
  • cosmology: observations
  • galaxies: evolution
  • galaxies: high-redshift
  • stars: massive

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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