The MOSDEF-LRIS Survey: The Interplay Between Massive Stars and Ionized
  Gas in High-Redshift Star-Forming Galaxies
  We present a joint analysis of rest-UV and rest-optical spectra obtained
using Keck/LRIS and Keck/MOSFIRE for a sample of 62 star-forming galaxies at
$z\sim2.3$. We divide our sample into 2 bins based on their location in the
[OIII]/Hb vs. [NII]/Ha BPT diagram, and perform the first differential study of
the rest-UV properties of massive ionizing stars as a function of rest-optical
emission-line ratios. Fitting BPASS stellar population synthesis models,
including nebular continuum emission, to our rest-UV spectra, we find that
high-redshift galaxies offset towards higher [OIII]/Hb and [NII]/Ha have
younger ages (log(Age/yr)=$7.20^{+0.57}_{-0.20}$) and lower stellar
metallicities ($Z_*=0.0010^{+0.0011}_{-0.0003}$) resulting in a harder ionizing
spectrum, compared to the galaxies in our sample that lie on the local BPT
star-forming sequence (log(Age/yr)=$8.57^{+0.88}_{-0.84}$,
$Z_*=0.0019\pm0.0006$). Additionally, we find that the offset galaxies have an
ionization parameter of log(U)=$-3.04^{+0.06}_{-0.11}$ and nebular metallicity
of 12+log(O/H)=$8.40^{+0.06}_{-0.07}$, and the non-offset galaxies have
log(U)=$-3.11\pm0.08$ and 12+log(O/H)=$8.30^{+0.05}_{-0.06}$. The stellar and
nebular metallicities derived for our sample imply that the galaxies offset
from the local BPT relation are more alpha-enhanced ($7.28^{+2.52}_{-2.82}$
O/Fe$_\odot$) compared to those consistent with the local sequence
($3.04^{+0.95}_{-0.54}$ O/Fe$_\odot$). However, even galaxies that are entirely
consistent with the local nebular excitation sequence appear to be
alpha-enhanced -- in contrast with typical local systems. Such differences must
be considered when estimating gas-phase O/H at high redshift based on strong
emission-line ratios. Specifically, a similarity in the location of
high-redshift and local galaxies in the BPT diagram may not be indicative of a
similarity in their physical properties.
