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

ABSTRACT We present a spectroscopic analysis of two star-forming galaxies at $z\simeq 5$ observed with JWST/NIRSpec as part of the Early eXtragalactic Continuum and Emission Line Science survey. The detection of the C iii] $\lambda$$\lambda$1906,09, [O ii] $\lambda$$\lambda$3726,29, [O iii] $\lambda$$\lambda$4363,5007, and [N ii] $\lambda$6584 emission lines enables an investigation of the $\mathrm{C/O}$, $\mathrm{N/O}$, and $\mathrm{C/N}$ abundance ratios using the temperature-sensitive method. The galaxies have stellar masses of ${\mathrm{log}(M_{\star }/\mathrm{M}_{\odot }) = 8.09^{+\, 0.24}_{-0.15}}$ and ${\mathrm{log}(M_{\star }/\mathrm{M}_{\odot }) = 8.02^{+\, 0.06}_{-0.08}}$ with metallicities of $Z \simeq 0.2 \, \rm {Z_{\odot }}$ and $Z \simeq 0.3 \, \rm {Z_{\odot }}$. These metallicities are somewhat higher than is typical for other $z\gtrsim 5$ galaxies with similar stellar mass and are comparable to $z \simeq 0$ analogues. Both galaxies display evidence for elevated N/O ratios with respect to the typical star-forming galaxies at $z\simeq 0$, with ${\mathrm{log(N/O)} = -1.07^{+\, 0.17}_{-0.13}}$ and ${\mathrm{log(N/O)} = -0.86^{+\, 0.15}_{-0.11}}$, respectively. In contrast, we find low C abundances, with ${\mathrm{log(C/O)}=-0.82\pm 0.22}$ and ${\mathrm{log(C/O)}=-1.02\pm 0.22}$, consistent with the predicted yields of core-collapse supernovae. Following the trend observed in other high-redshift sources, we find that the $\mathrm{C/N}$ ratios are lower at fixed $\mathrm{O/H}$ 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 ($\simeq 4\!-\!7 \, \mathrm{M}_{\odot }$) 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.