Excitation and Ionization Properties of Star-forming Galaxies at z=2.0-9.3 with JWST/NIRSpec

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 [O III]/H beta versus [N II]/H alpha, [S II]/Ha, and [O I]/Ha "BPT" diagrams at z > 2.7, and the first analysis of the O-32 versus R-23 diagram at z > 4 with a large sample. We divide the sample into five redshift bins containing 30-40 galaxies each. The subsamples at z similar to 2.3, z similar to 3.3, and z similar to 4.5 are representative of the main-sequence star-forming galaxy population at these redshifts, while the z similar to 5.6 and z similar to 7.5 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 [N II] and [S II] BPT diagrams and the O-32-R-23 diagram on average, and is offset from the sequences followed by z = 0 H II 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 z similar to 2 and z similar to 6. Overall, the rest-optical line ratios suggest the z = 2.7-9.3 CEERS/NIRSpec galaxies at log(M-*/M-circle dot) similar to 7.5-10 have high degrees of ionization and moderately low oxygen abundances (similar to 0.1-0.3 Z(circle dot)), but are not extremely metal-poor (<0.1 Z(circle dot)) even at z > 6.5.