The nebular properties of star-forming galaxies at intermediate redshift
  from the Large Early Galaxy Astrophysics Census
  We present a detailed study of the partial rest-optical
($\lambda_{\mathrm{obs}} \approx 3600-5600\,\r{A}$) spectra of $N = 328$
star-forming galaxies at $0.6 < z < 1.0$ from the Large Early Galaxy
Astrophysics Census (LEGA-C). We compare this sample with low-redshift ($z \sim
0$) galaxies from the Sloan Digital Sky Survey (SDSS), intermediate-redshift
($z \sim 1.6$) galaxies from the Fiber Multi-Object Spectrograph (FMOS)-COSMOS
Survey, and high-redshift ($z \sim 2$) galaxies from the Keck Baryonic
Structure Survey (KBSS). At a lookback time of $6-8\ \mathrm{Gyr}$, galaxies
with stellar masses $\mathrm{log}(\mathrm{M_{\ast}/M_{\odot}}) > 10.25$ appear
remarkably similar to $z \sim 0$ galaxies in terms of their nebular excitation,
as measured using $\mathrm{[O\,III]}\lambda5008 / \mathrm{H}\beta$. There is
some evidence that $0.6 < z < 1.0$ galaxies with lower $\mathrm{M_{\ast}}$ have
higher $\mathrm{[O\,III]}\lambda5008 / \mathrm{H}\beta$ than $z \sim 0$
galaxies and are more similar to less evolved $z \sim 1.6$ and $z \sim 2$
galaxies, which are offset from the $z \sim 0$ locus at all
$\mathrm{M_{\ast}}$. We explore the impact selection effects, contributions
from active galactic nuclei, and variations in physical conditions (ionization
parameter and gas-phase oxygen abundance) have on the apparent distribution of
$\mathrm{[O\,III]}\lambda5008 / \mathrm{H}\beta$ and find somewhat higher
ionization and lower enrichment in $0.6 < z < 1.0$ galaxies with lower
$\mathrm{M_{\ast}}$ relative to $z \sim 0$ galaxies. We use new near-infrared
spectroscopic observations of $N = 53$ LEGA-C galaxies to investigate other
probes of enrichment and excitation. Our analysis demonstrates the importance
of obtaining complete rest-optical spectra of galaxies in order to disentangle
these effects.
