  We present a combined analysis of rest-frame far-UV (1000-2000 A) and
rest-frame optical (3600-7000 A) composite spectra formed from very deep
observations of a sample of 30 star-forming galaxies with z=2.4+/-0.1, selected
to be representative of the full KBSS-MOSFIRE spectroscopic survey. Since the
same massive stars are responsible for the observed FUV continuum and the
excitation of the observed nebular emission, a self-consistent stellar
population synthesis model must simultaneously match the details of the far-UV
stellar+nebular continuum and-- when inserted as the excitation source in
photoionization models-- account for all observed nebular emission line ratios.
We find that only models including massive star binaries, having low stellar
metallicity (Z_*/Z_{sun} ~ 0.1) but relatively high ionized gas-phase oxygen
abundances (Z_{neb}/Z_{sun} ~ 0.5), can successfully match all of the
observational constraints. We argue that this apparent discrepancy is naturally
explained by highly super-solar O/Fe [4-5 times (O/Fe)_{sun}], expected for gas
whose enrichment is dominated by the products of core-collapse supernovae. Once
the correct ionizing spectrum is identified, photoionization models reproduce
all of the observed strong emission line ratios, the direct T_e measurement of
O/H, and allow accurate measurement of the gas-phase abundance ratios of N/O
and C/O -- both of which are significantly sub-solar but, as for O/Fe, are in
remarkable agreement with abundance patterns observed in Galactic thick disk,
bulge, and halo stars with similar O/H. High nebular excitation is the rule at
high-z (and rare at low-z) because of systematically shorter enrichment
timescales (<<1 Gyr): low Fe/O environments produce harder (and longer-lived)
stellar EUV spectra at a given O/H, enhanced by dramatic effects on the
evolution of massive star binaries.
