CLEAR: Emission-line Ratios at Cosmic High Noon

We use Hubble Space Telescope Wide Field Camera 3 G102 and G141 grism spectroscopy to measure rest-frame optical emission-line ratios of 533 galaxies at z ~ 1.5 in the CANDELS Lyα Emission at Reionization survey. We compare [O III]/Hβ versus [S II]/(Hα + [N II]) as an "unVO87" diagram for 461 galaxies and [O III]/Hβ versus [Ne III]/[O II] as an "OHNO" diagram for 91 galaxies. The unVO87 diagram does not effectively separate active galactic nuclei (AGN) and [Ne V] 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 does effectively separate X-ray AGN and [Ne V]-emitting galaxies from the rest of the population. We find that the [O III]/Hβ line ratios are significantly anticorrelated with stellar mass and significantly correlated with $\mathrm{log}({L}_{{\rm{H}}\beta })$ , while [S II]/(Hα + [N II]) is significantly anticorrelated with $\mathrm{log}({L}_{{\rm{H}}\beta })$ . Comparison with MAPPINGS V photoionization models indicates that these trends are consistent with lower metallicity and higher ionization in low-mass and high-star formation rate (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 [O III]/Hβ versus [Ne III]/[O II] 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.