CEERS Key Paper. VIII. Emission-line Ratios from NIRSpec and NIRCam Wide-Field Slitless Spectroscopy at z > 2

We use James Webb Space Telescope Near-Infrared Camera Wide Field Slitless Spectroscopy (NIRCam WFSS) and the Near-Infrared spectrograph (NIRSpec) in the Cosmic Evolution Early Release survey to measure rest-frame optical emission-line ratios of 155 galaxies at z > 2. The blind NIRCam grism observations include a sample of galaxies with bright emission lines that were not observed on the NIRSpec masks. We study the changes of the H alpha, [O III]/H beta, and [Ne III]/[O II] emission lines in terms of redshift by comparing to lower-redshift SDSS, CLEAR, and MOSDEF samples. We find a significant (>3 sigma) correlation between [O III]/H beta with redshift, while [Ne III]/[O II] has a marginal (2 sigma) correlation with redshift. We compare [O III]/H beta and [Ne III]/[O II] to stellar mass and H beta SFR. We find that both emission-line ratios have a correlation with H beta SFR and an anticorrelation with stellar mass across the redshifts 0 < z < 9. Comparison with MAPPINGS V models indicates that these trends are consistent with lower metallicity and higher ionization in low-mass and high-SFR galaxies. We additionally compare to IllustrisTNG predictions and find that they effectively describe the highest [O III]/H beta ratios observed in our sample, without the need to invoke MAPPINGS models with significant shock ionization components.