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

ABSTRACT We present a spectroscopic analysis of two star-forming galaxies at z 5 observed with JWST/NIRSpec as part of the Early eXtragalactic Continuum and Emission Line Science survey. The detection of the C iii] 1906, 09, O ii 3726, 29, O iii 4363, 5007, and N ii 6584 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 initial mass function 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.