JWST/NIRSpec Measurements of the Relationships between Nebular Emission-line Ratios and Stellar Mass at z ∼ 3–6
Alice E. Shapley, Naveen A. Reddy, Ryan L. Sanders, Michael W. Topping, Gabriel Brammer
Abstract
Abstract 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 ] λ 6583 feature ([N ii ] λ 6583/H α , ([O iii ] λ 5007/H β )/([N ii ] λ 6583/H α ) (O3N2), and [N ii ] λ 6583/[O ii ] λ 3727) and those featuring α -elements ([O iii ] λ 5007/H β , [O iii ] λ 5007/[O ii ] λ 3727 (O 32 ), ([O iii ] λλ 4959, 5007 + [O ii ] λ 3727)/H β (R 23 ), and [Ne iii ] λ 3869/[O ii ] λ 3727). Given the typical flux levels of [N ii ] λ 6583 and [Ne iii ] λ 3869, 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 ] λ 5007/H β , O 32 , O3N2) and weaker nitrogen emission (e.g., lower [N ii ] λ 6583/H α and [N ii ] λ 6583/[O ii ] λ 3727) 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 ] λ 3869/[O ii ] λ 3727 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.