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We utilize medium-resolution JWST/NIRSpec observations of 164 galaxies at z=2. 0-9. 3 from the Cosmic Evolution Early Release Science (CEERS) survey to investigate the evolution of the excitation and ionization properties of galaxies at high redshifts. Our results represent the first statistical constraints on the evolution of the OIII/Hβ vs. NII/Hα, SII/Hα, and OI/Hα ``BPT'' diagrams at z>2. 7, and the first analysis of the O32 vs. R23 diagram at z>4 with a large sample. We divide the sample into five redshift bins containing 30-40 galaxies each. The subsamples at z2. 3, z3. 3, and z4. 5 are representative of the main-sequence star-forming galaxy population at these redshifts, while the z5. 6 and z7. 5 samples are likely biased toward high specific star-formation rate due to selection effects. Using composite spectra, we find that each subsample at z=2. 0-6. 5 falls on the same excitation sequence in the NII and SII BPT diagrams and the O32-R23 diagram on average, offset from the sequences followed by z=0 HII regions in the same diagrams. The direction of these offsets are consistent with high-redshift star-forming galaxies uniformly having harder ionizing spectra than typical local galaxies at fixed nebular metallicity. The similarity of the average line ratios suggests that the ionization conditions of the interstellar medium do not strongly evolve between z2 and z6. Overall, the rest-optical line ratios suggest the z=2. 7-9. 3 CEERS/NIRSpec galaxies at log (M_*/M_) 7. 5-10 have high degrees of ionization and moderately low oxygen abundances (0. 1-0. 3~Z_), but are not extremely metal poor (6. 5.
Sanders et al. (Tue,) studied this question.