CLEAR: Emission Line Ratios at Cosmic High Noon

We use \textit{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 $\OIII/\Hb$ vs. $\SII/\HavNII$ as an "unVO87" diagram for 461 galaxies and $\OIII/\Hb$ vs. $\NeIII/\OII$ 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 $\OIII/\Hb$ line ratios are significantly anti-correlated with stellar mass and significantly correlated with $\log(L_{\Hb})$, while $\SII/\HavNII$ is significantly anti-correlated with $\log(L_{\Hb})$. 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 $\OIII/\Hb$ vs. $\NeIII/\OII$ will be a useful indicator of AGN content and gas conditions in very high-redshift galaxies to be observed by the \textit{James Webb Space Telescope}.