Reconciling the stellar and nebular spectra of high-redshift galaxies

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 (Z_neb/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.