CLEAR: Emission Line Ratios at Cosmic High Noon
  We use Hubble Space Telescope WFC3 G102 and G141 grism spectroscopy to
measure rest-optical emission-line ratios of 533 galaxies at $z\sim1.5$ in the
CANDELS Ly$\alpha$ Emission at Reionization (CLEAR) survey. We compare
$\frac{[OIII]}{H\beta}$ vs. $\frac{[SII]}{(H\alpha +[NII])}$ as an "unVO87"
diagram for 461 galaxies and $\frac{[OIII]}{Hb}$ vs. $\frac{[NeIII]}{[OII]}$ as
an "OHNO" diagram for 91 galaxies. The unVO87 diagram does not effectively
separate active galactic nuclei (AGN) and $[NeV]$ sources from star-forming
galaxies, indicating that the unVO87 properties of star-forming galaxies evolve
with redshift and overlap with AGN emission-line signatures at $z>1$. The OHNO
diagram effectively separates X-ray AGN and $[NeV]$-emitting galaxies from the
rest of the population. We find that the $\frac{[OIII]}{H\beta}$ line ratios
are significantly anti-correlated with stellar mass and significantly
correlated with $\log(L_{H\beta})$, while $\frac{[SII]}{(H\alpha +[NII])}$ is
significantly anti-correlated with $\log(L_{H\beta})$. Comparison with
MAPPINGS~V photoionization models indicates that these trends are consistent
with lower metallicity and higher ionization in low-mass and high-SFR galaxies.
We do not find evidence for redshift evolution of the emission-line ratios
outside of the correlations with mass and SFR.Our results suggest that the OHNO
diagram of $\frac{[OIII]}{Hb}$ vs. $\frac{[NeIII]}{[OII]}$ will be a useful
indicator of AGN content and gas conditions in very high-redshift galaxies to
be observed by the James Webb Space Telescope.
