Emission-line Diagnostics at z  ≳  2: A Probe of the Ionizing Spectrum and α -enhancement beyond Cosmic Noon

We analyze several key rest-optical emission-line ratios in a sample of 828 galaxies as well as composite spectra from JADES DR3 in the range 1.4 < z < 7. These emission-line ratios include: [O iii ] λ 5008/H β , [N ii ] λ 6585/H α , [S ii ] λλ 6718, 6733/H α , [O i ] λ 6302/H α , O32, R23, Ne3O2, and RO2Ne3. We find evidence for a harder ionizing spectrum at z ∼ 3.5 compared to z ∼ 2 at fixed gas-phase metallicity, resulting in a pronounced shift in the star-forming galaxy locus on the [N ii ]/H α Baldwin-Phillips-Terlevich (BPT) diagram and the O32 versus R23 diagram. At z ≳ 3.5, star-forming galaxies occupy a common locus, indicating that interstellar medium (ISM) ionizing conditions at fixed gas-phase metallicity do not evolve strongly at these early cosmic times. There is a connection between ISM ionizing conditions and the chemical abundance patterns (i.e., α /Fe) in massive stars, providing the ionizing radiation field. Therefore, the lack of evolution in ISM ionizing conditions at z ≳ 3.5, followed by evolution towards a softer ionizing spectrum at fixed nebular metallicity as cosmic time proceeds to z ∼ 2 and lower redshift mirrors the chemical abundance patterns in Milky Way stars as a function of iron abundance. Our results highlight the diagnostic power of emission-line diagrams in the era of JWST to further our understanding of the ISM conditions into the Epoch of Reionization.