2023ApJ...950L...1S

We analyze the rest-optical emission-line ratios of star-forming galaxies at 2.7 ≤ z < 6.5 drawn from the Cosmic Evolution Early Release Science (CEERS) Survey and their relationships with stellar mass (M_*). Our analysis includes both line ratios based on the [N II]λ6583 feature ([N II]λ6583/Hα, ([O III]λ5007/Hβ)/([N II]λ6583/Hα) (O3N2), and [N II]λ6583/[O II]λ3727) and those featuring α-elements ([O III]λ5007/Hβ, [O III]λ5007/[O II]λ3727 (O₃₂), ([O III]λλ4959, 5007 + [O II]λ3727)/Hβ (R₂₃), and [Ne III]λ3869/[O II]λ3727). Given the typical flux levels of [N II]λ6583 and [Ne III]λ3869, which are undetected in the majority of individual CEERS galaxies at 2.7 ≤ z < 6.5, we construct composite spectra in bins of M_* and redshift. Using these composite spectra, we compare the relationships between emission-line ratios and M_* at 2.7 ≤ z < 6.5 with those observed at lower redshift. While there is significant evolution toward higher excitation (e.g., higher [O III]λ5007/Hβ, O₃₂, O3N2) and weaker nitrogen emission (e.g., lower [N II]λ6583/Hα and [N II]λ6583/[O II]λ3727) between z ∼ 0 and z ∼ 3, we find in most cases that there is no significant evolution in the relationship between line ratio and M_* beyond z ∼ 3. The [Ne III]λ3869/[O II]λ3727 ratio is anomalous in showing evidence for significant elevation at 4.0 ≤ z < 6.5 at fixed mass, relative to z ∼ 3.3. Collectively, however, our empirical results suggest no significant evolution in the mass-metallicity relationship at 2.7 ≤ z < 6.5. Representative galaxy samples and metallicity calibrations based on existing and upcoming JWST/NIRSpec observations will be required to translate these empirical scaling relations into ones tracing chemical enrichment and gas cycling and to distinguish among descriptions of feedback in galaxy formation simulations at z > 3.