The JWST EXCELS survey: direct estimates of C, N, and O abundances in two relatively metal-rich galaxies at z ≃ 5

We present a spectroscopic analysis of two star-forming galaxies at 𝑧 ≃ 5 observed with JWST/NIRSpec as part of the EXCELS survey. The detection of the Ciii] 𝜆𝜆1906,09, [O ii] 𝜆𝜆3726,29, [O iii] 𝜆𝜆4363,5007, and [N ii] 𝜆6584 emission lines enables an investigation of the C/O, N/O, and C/N abundance ratios using the temperature-sensitive method. The galaxies have stellar masses of log(𝑀★/M⊙) = 8.09+ 0.24
−0.15 and log(𝑀★/M⊙) = 8.02+ 0.06−0.08 with metallicities of 𝑍 ≃ 0.2 Z⊙ and 𝑍 ≃ 0.3 Z⊙. These metallicities are somewhat higher than is typical for other 𝑧 ≳ 5 galaxies with similar stellar mass and are comparable to 𝑧 ≃ 0 analogues. Both
galaxies display evidence for elevated N/O ratios with respect to the typical star-forming galaxies at 𝑧 ≃ 0, with log(N/O) = −1.07+ 0.17 −0.13 and log(N/O) = −0.86+ 0.15
−0.11 respectively. In contrast, we find low C abundances, with log(C/O) = −0.82 ± 0.22 and log(C/O) = −1.02 ± 0.22, consistent with the predicted yields of core-collapse supernovae. Following the trend observed in other high-redshift sources, we find that the C/N ratios are lower at fixed O/H compared to the majority of local galaxies. Via a comparison to detailed chemical evolution models, we find that a standard or bottom-heavy IMF can explain the observed abundance ratios where the
N-enrichment comes from intermediate mass (≃ 4 − 7 M⊙) stars. Our results demonstrate that robust measurements of CNO abundances with JWST can reveal unique enrichment pathways in galaxies as a function of both metallicity and redshift.