Discovery of an Extremely Metal-Poor Galaxy at $z=3.654$ Using JWST Infrared Spectroscopy

TL;DR

This study reports the discovery of CAPERS-39810, an extremely metal-poor galaxy at $z=3.654$, identified with JWST/NIRSpec prism spectroscopy showing $12+\log(O/H)=6.73\pm0.13$. The authors combine strong-line R3 metallicity calibration with Bagpipes-based SED modeling of COSMOS2025 photometry to derive a stellar mass of $\log_{10}(M_*/M_\odot)=8.02^{+0.22}_{-0.34}$, an age of $0.27^{+0.33}_{-0.20}$ Gyr, and a star formation rate of $0.26^{+0.23}_{-0.12}$ M$_\odot$ yr$^{-1}$. CAPERS-39810 lies significantly below the $M_*/Z$ relation at $z\sim3$, extending the population of extremely metal-poor galaxies to intermediate redshift and implying that primitive chemical enrichment persists at cosmic noon. Together, these results enhance our understanding of chemical enrichment pathways and star formation in low-mass galaxies across cosmic time, aided by JWST's capability to detect faint EMPGs at high redshift.

Abstract

We report the discovery of an extremely metal-poor galaxy at a redshift of z = 3.654, identified through infrared spectroscopy using the James Webb Space Telescope (JWST). This galaxy, CAPERS-39810, exhibits a metallicity of 12 + log(O/H) = $6.73\pm0.13$, indicative of its primitive chemical composition, resembling the early stages of galaxy formation in the Universe. We use JWST NIRSpec/MSA for spectroscopic analysis, complemented by photometric data from the COSMOS2025 catalog. Our analysis employs the R3 strong-line diagnostic method to estimate metallicity, due to the lack of auroral lines in the spectrum. The galaxy's emission lines, including Hb, [O III], Ha and He I, are clearly detected. The rest-frame equivalent widths of the strong hydrogen recombination lines are EW_0(Hb) = $184\pm48$ Åand EW_0(Ha) = $1144\pm48$ Å. Furthermore, we perform detailed spectral energy distribution modeling to derive a galaxy logarithmic stellar mass of $8.02^{+0.22}_{-0.34}$ $M_\odot$. This discovery adds to the growing body of evidence for the existence of very low-metallicity galaxies existed at cosmic noon of $z\approx3$, which are crucial for understanding the processes of chemical enrichment and star formation in young galaxies at the cosmic noon.

Figures (4)

• Figure 1: JWST observations of CAPERS-39810 are shown. The left panel displays a composite RGB image constructed from JWST NIRCam imaging, where the F444W, F277W, and the average of the F115W and F150W bands are mapped to the red, green, and blue channels, respectively. The white polygon denotes the configuration of the NIRSpec shutters. The right panel shows the stacked NIRSpec/Prism 2D spectrum together with the extracted 1D spectrum. The spectral extraction region is indicated by the white dashed lines. The black solid line and the red dashed line correspond to the measured flux and the associated uncertainty. Key emission features, including H$\beta$, H$\alpha$, He i$\lambda$10830, and the metal line [O iii] $\lambda\lambda$4959, 5007, are identified.
• Figure 2: SED modeling of the CAPERS-39810. The upper panel shows the results of SED fitting to the JWST NIRCam photometry. The red points represent the observed photometric measurements, while the orange curve denotes the best-fitting model SED. The blue squares overlaid on the model indicate the synthetic photometry computed from the best-fit model. The lower panel presents the posterior probability distributions of the SED model parameters. The black dashed lines mark the 16th, 50th, and 84th percentiles of the posterior distributions.
• Figure 3: Gas-phase metallicity of the CAPERS-39810. For comparison, the gray markers in the figure denote the NIRSpec samples for which metallicities have been determined using the direct $T_{e}$ method (Sanders2020MNRAS.491.1427S; Sanders2024ApJ...962...24S; Chakraborty2025ApJ...985...24C; Scholte2025MNRAS.540.1800S), whereas the recently reported metal-poor galaxies are highlighted with colored markers (Cai2025ApJ...993L..52C; Nakajima2025arXiv250611846N; Vanzella2025arXiv250907073V; Hsiao2025arXiv250503873H; Willott2025ApJ...988...26W; Cullen2025MNRAS.540.2176C; Morishita2025arXiv250710521M; Mondal2025ApJ...988..171M). All upper limits in the figure are 1$\sigma$ values.
• Figure 4: The stellar mass-metallicity relation. Colored symbols denote candidate extremely metal-poor systems, corresponding to the sources highlighted in Fig. \ref{['fig:Z-z']}. The black dashed, brown solid, and dark-green dot-dashed curves show the best-fit MZR relations at $z\sim3$ from Curti2024AA...684A..75C, Li2023ApJ...955L..18L, and Sanders2021ApJ...914...19S, respectively, with their 1$\sigma$ uncertainties indicated by the shaded regions; the dark-green dotted lines denote the extrapolation of the Sanders2021ApJ...914...19S relation. The overplotted gray squares and brown triangles show galaxy samples from the JWST JADES and GLASS surveys.