The JWST EXCELS survey: direct estimates of C, N, and O abundances in two relatively metal-rich galaxies at $\mathbf{z}\simeq 5$

Abstract

We present a spectroscopic analysis of two star-forming galaxies at $z\simeq 5$ observed with JWST/NIRSpec as part of the EXCELS survey. The detection of the CIII]~$\lambda \lambda$1906,09, [OII] $\lambda \lambda$3726,29, [OIII] $\lambda \lambda$4363,5007, and [NII] $\lambda$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 $\log (M_{\star }/{\mathrm{M}}_{\odot })=8.09_{-0.15}^{+0.24}$ and $\log (M_{\star }/{\mathrm{M}}_{\odot })=8.02_{-0.08}^{+0.06}$ with metallicities of $Z\simeq 0.2{\mathrm{Z}}_{\odot }$ and $Z\simeq 0.3{\mathrm{Z}}_{\odot }$. 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 $\log (N/O)=-1.07_{-0.13}^{+0.17}$ and $\log (N/O)=-0.86_{-0.11}^{+0.15}$ respectively. In contrast, we find low C abundances, with $\log (C/O)=-0.82\pm 0.22$ and $\log (C/O)=-1.02\pm 0.22$, consistent with the predicted yields of core-collapse supernovae. Following the trend observed in other high-redshift sources, we find that the $C/N$ 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 ($\simeq 4-7{\mathrm{M}}_{\odot }$) stars. Our results demonstrate that robust measurements of CNO abundances with \emph{JWST} can reveal unique enrichment pathways in galaxies as a function of both metallicity and redshift.

17 pages, 8 Figures, and 3 Tables. Accepted in MNRAS