The MOSDEF Survey: Excitation Properties of $z\sim 2.3$ Star-forming
  Galaxies
  We present results on the excitation properties of z~2.3 galaxies using early
observations from the MOSFIRE Deep Evolution Field (MOSDEF) Survey. With its
coverage of the full suite of strong rest-frame optical emission lines, MOSDEF
provides an unprecedented view of the rest-frame optical spectra of a
representative sample of distant star-forming galaxies. We investigate the
locations of z~2.3 MOSDEF galaxies in multiple emission-line diagnostic
diagrams. These include the [OIII]/Hb vs. [NII]/Ha and [OIII]/Hb vs. [SII]/Ha
"BPT" diagrams, as well as the O_32 vs. R_23 excitation diagram. We recover the
well-known offset in the star-forming sequence of high-redshift galaxies in the
[OIII]/Hb vs. [NII]/Ha BPT diagram relative to SDSS star-forming galaxies.
However, the shift for our rest-frame optically selected sample is less
significant than for rest-frame-UV selected and emission-line selected galaxies
at z~2. Furthermore, we find that the offset is mass-dependent, only appearing
within the low-mass half of the z~2.3 MOSDEF sample, where galaxies are shifted
towards higher [NII]/Ha at fixed [OIII]/Hb. Within the [OIII]/Hb vs. [SII]/Ha
and O_32 vs. R_23 diagrams, we find that z~2.3 galaxies are distributed like
local ones, and therefore attribute the shift in the [OIII]/Hb vs. [NII]/Ha BPT
diagram to elevated N/O abundance ratios among lower-mass (M_*<10^10 M_sun)
high-redshift galaxies. The variation in N/O ratios calls into question the use
at high redshift of oxygen abundance indicators based on nitrogen lines, but
the apparent invariance with redshift of the excitation sequence in the O_32
vs. R_23 diagram paves the way for using the combination of O_32 and R_23 as an
unbiased metallicity indicator over a wide range in redshift. This indicator
will allow for an accurate characterization of the shape and normalization of
the mass-metallicity relationship over more than 10 Gyr.
