The MOSDEF-LRIS Survey: The Connection Between Massive Stars and Ionized
  Gas in Individual Galaxies at $z\sim2$
  We present constraints on the massive star and ionized gas properties for a
sample of 62 star-forming galaxies at $z\sim2.3$. Using BPASS stellar
population models, we fit the rest-UV spectra of galaxies in our sample to
estimate age and stellar metallicity which, in turn, determine the ionizing
spectrum. In addition to the median properties of well-defined subsets of our
sample, we derive the ages and stellar metallicities for 30 high-SNR individual
galaxies -- the largest sample of individual galaxies at high redshift with
such measurements. Most galaxies in this high-SNR subsample have stellar
metallicities of $0.001<Z_*<0.004$. We then use Cloudy+BPASS photoionization
models to match observed rest-optical line ratios and infer nebular properties.
Our high-SNR subsample is characterized by a median ionization parameter and
oxygen abundance, respectively, of $\log(U)_{\textrm{med}}=-2.98\pm0.25$ and
$12+\log(\textrm{O/H})_{\textrm{med}}=8.48\pm0.11$. Accordingly, we find that
all galaxies in our sample show evidence for $\alpha$-enhancement. In addition,
based on inferred $\log(U)$ and $12+\log(\textrm{O/H})$ values, we find that
the local relationship between ionization parameter and metallicity applies at
$z\sim2$. Finally, we find that the high-redshift galaxies most offset from the
local excitation sequence in the BPT diagram are the most $\alpha$-enhanced.
This trend suggests that $\alpha$-enhancement resulting in a harder ionizing
spectrum at fixed oxygen abundance is a significant driver of the high-redshift
galaxy offset on the BPT diagram relative to local systems. The ubiquity of
$\alpha$-enhancement among $z\sim2.3$ star-forming galaxies indicates important
differences between high-redshift and local galaxies that must be accounted for
in order to derive physical properties at high redshift.
