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A Metal-Free Galaxy at $z = 3.19$? Evidence of Late Population III Star Formation at Cosmic Noon

Sijia Cai, Mingyu Li, Zheng Cai, Yunjing Wu, Fujiang Yu, Mark Dickinson, Fengwu Sun, Xiaohui Fan, Ben Wang, Fergus Cullen, Fuyan Bian, Xiaojing Lin, Jiaqi Zou

TL;DR

The paper addresses whether metal-free Population III star formation can persist into the cosmic noon. It combines JWST/NIRSpec prism and VLT/MUSE spectroscopy with deep multi-band imaging to identify CR3, an extremely metal-poor galaxy at $z=3.193$ and to characterize its emission lines, metallicity, stellar population, and environment. The results show extremely strong recombination lines (Lyα, Hα, He I 10830), a stringent gas-phase metallicity upper limit of $12+\log(O/H) < 6.52$, and a young, low-mass Pop III–like stellar population ($t_{age} \approx 2$ Myr, $M_* \approx 6.1\times10^5\,M_⊙$); the galaxy resides in a mildly underdense region ($δ \approx -0.12$). Together, these findings imply that pristine gas pockets capable of Pop III star formation can survive to $z\sim3$, challenging the conventional view that such episodes ended by $z\gtrsim6$ and highlighting the importance of low-density environments for late-time Pop III activity.

Abstract

Star formation from metal-free gas, the hallmark of the first generation of Population III stars, was long assumed to occur only in the very early Universe. We report the discovery of MPG-CR3 (Metal-Pristine Galaxy COSMOS Redshift 3; hereafter CR3), an extremely metal-poor galaxy at redshift $z= 3.193\pm0.016$. From JWST, VLT, and Subaru observations, CR3 exhibits exceptionally strong Ly$α$, H$α$, and He I $λ$10830 emission. We measure rest-frame equivalent widths of EW$_0$(Ly$α$) $=822\pm101$ Angstrom and EW$_0$(H$α$) $=2814\pm327$ Angstrom, among the highest seen in star-forming systems. No metal lines, e.g. [O III] $λ\lambda4959,5007$, C IV $λ\lambda1548,1550$, have statistically significant detections, placing a 2-$σ$ upper limit on the gas-phase metallicity of 12+log(O/H) < 6.52 ($Z < 7\times10^{-3}\ Z_\odot$) with strong-line calibration established by JWST, making it the most metal-poor galaxy known at cosmic noon. Considering systematic uncertainties of $\gtrsim 0.3$ dex in the calibrations, the most conservative 2-$σ$ upper limit is set to 12+log(O/H) < 6.95. The observed Ly$α$/H$α$ flux ratio is $13.9\pm2.5$, indicating negligible dust attenuation. Spectral energy distribution modeling with Pop III stellar templates indicates a very young ($\sim2$ Myr), low-mass ($M_* \approx 6.1\times 10^5 M_\odot$) stellar population. Further, the photometric redshifts reveal that CR3 could reside in a slightly underdense environment ($δ\approx -0.12$). CR3 provides evidence that first-generation star formation could persist well after the epoch of reionization, challenging the conventional view that pristine star formation ended by $z\gtrsim6$.

A Metal-Free Galaxy at $z = 3.19$? Evidence of Late Population III Star Formation at Cosmic Noon

TL;DR

The paper addresses whether metal-free Population III star formation can persist into the cosmic noon. It combines JWST/NIRSpec prism and VLT/MUSE spectroscopy with deep multi-band imaging to identify CR3, an extremely metal-poor galaxy at and to characterize its emission lines, metallicity, stellar population, and environment. The results show extremely strong recombination lines (Lyα, Hα, He I 10830), a stringent gas-phase metallicity upper limit of , and a young, low-mass Pop III–like stellar population ( Myr, ); the galaxy resides in a mildly underdense region (). Together, these findings imply that pristine gas pockets capable of Pop III star formation can survive to , challenging the conventional view that such episodes ended by and highlighting the importance of low-density environments for late-time Pop III activity.

Abstract

Star formation from metal-free gas, the hallmark of the first generation of Population III stars, was long assumed to occur only in the very early Universe. We report the discovery of MPG-CR3 (Metal-Pristine Galaxy COSMOS Redshift 3; hereafter CR3), an extremely metal-poor galaxy at redshift . From JWST, VLT, and Subaru observations, CR3 exhibits exceptionally strong Ly, H, and He I 10830 emission. We measure rest-frame equivalent widths of EW(Ly) Angstrom and EW(H) Angstrom, among the highest seen in star-forming systems. No metal lines, e.g. [O III] , C IV , have statistically significant detections, placing a 2- upper limit on the gas-phase metallicity of 12+log(O/H) < 6.52 () with strong-line calibration established by JWST, making it the most metal-poor galaxy known at cosmic noon. Considering systematic uncertainties of dex in the calibrations, the most conservative 2- upper limit is set to 12+log(O/H) < 6.95. The observed Ly/H flux ratio is , indicating negligible dust attenuation. Spectral energy distribution modeling with Pop III stellar templates indicates a very young ( Myr), low-mass () stellar population. Further, the photometric redshifts reveal that CR3 could reside in a slightly underdense environment (). CR3 provides evidence that first-generation star formation could persist well after the epoch of reionization, challenging the conventional view that pristine star formation ended by .

Paper Structure

This paper contains 13 sections, 2 equations, 4 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Observations
  3. Imaging Data
  4. Spectroscopic Data
  5. Methods and Analysis
  6. Emission Lines
  7. Gas-phase Metallicity
  8. Photometry and SED Modeling
  9. Large-scale Environment
  10. Results and Discussions
  11. A Pop III Candidate Growing in a Cosmic Noon Underdense Environment
  12. Current non-detection of He2 Emission Lines
  13. Conclusions

Figures (4)

  • Figure 1: JWST and VLT observation of the CR3.Upper left: Pseudo-color composite image constructed from JWST/NIRCam images, using blue (F115W, F150W), green (F200W, F277W), and red (F356W, F444W) channels. The orientation of the MSA shutters is indicated by the white region. Upper right: JWST/NIRSpec prism 2D and 1D spectra. The red dashed lines indicate the optimal extraction aperture on the 2D spectrum. In the 1D panel, the black solid line shows the flux, while the red dotted line shows the associated error. The H$\alpha$ and He1$\,\lambda10830$ emission features are highlighted with blue shaded regions. Lower: 1D VLT/MUSE spectrum extracted from a circular aperture, rebinned and Hanning-smoothed. The black solid line shows the flux, and the red dotted line indicates the error. The Subaru/HSC $r$-band continuum level is overplotted as a gray dashed line. The Ly$\alpha$ emission line is highlighted with a blue shaded region. Strong sky emission lines are marked with light gray shaded areas. Hydrogen and helium emission lines are labeled in blue, while metal lines are labeled in red.
  • Figure 2: Multi-wavelength images and SED modeling of the CR3.Upper: Cutout stamps of the CR3 from Subaru/Suprime IA505, Subaru/HSC $g$, $r$, $i$, and JWST/NIRCam F090W, F115W, F150W, F200W, F277W, F356W, F410M, and F444W. The cutout image size is $4^{\prime\prime}$ for Subaru and $1^{\prime\prime}$ for JWST. Lower: Spectral energy distribution (SED) modeling of the CR3. The best-fit Pop III SED model is shown as a black curve, with model photometry (blue squares) and observed photometry (blue circles) overlaid. The 2$\sigma$ upper limits in the U, B, $z$, and $y$ bands are indicated by red arrows. The Ly$\alpha$ emission boosts the fluxes in the IA505 and HSC-$g$ bands; H$\alpha$ emission boosts the F277W band; and He1 emission enhances the F444W band.
  • Figure 3: Constraints on the gas-phase metallicity of the CR3.Left: We apply the Sanders2024ApJ...962...24S strong-line calibration to the measured upper limit on the R3 ratio ([O3]$\,\lambda5007$/H$\beta$; blue curve). For comparison, metallicity calibration curves from Hirschmann2023MNRAS.526.3504H and Nakajima2022ApJS..262....3N are presented. The dash-dotted extensions indicate the extrapolated regimes of the calibrations. Our measurement is compared to other known spectroscopically confirmed Pop III candidates or extremely metal-poor galaxies (EMPGs), including the LAP1 stellar complex (orange hexagon, Vanzella2023AA...678A.173V), the LAP1-B clump (orange hexagon, Nakajima2025arXiv250611846N), the LAP2-b stellar complex (yellow square, Vanzella2025arXiv250907073V), the AMORE6-A+B (gray diamond, Morishita:2025zvd), the GNHeII J1236+6215 (green circle, Mondal2025arXiv250606831M), the CANUCS-A370-z8-LAE (purple pentagon, Willott2025arXiv250207733W), the EXCELS-63107 (pink triangle, Cullen2025MNRAS.540.2176C) and the SAPPHIRES extremely metal-poor galaxies (blue diamond, Hsiao2025arXiv250503873H). Light gray markers show samples from the MOSDEF Sanders2020MNRAS.491.1427S, the CEERS Sanders2024ApJ...962...24S, the JADES and the PRIMAL Chakraborty2025ApJ...985...24C, and the EXCELS Scholte2025MNRAS.540.1800S. Right: Redshift–metallicity distribution of the sample, highlighting the extremely low metallicity of the CR3 compared to other galaxies at similar and higher redshifts. All limits shown represent 1-$\sigma$ upper bounds. The red rectangle indicates the systematic uncertainty associated with the strong-line calibration discussed in Section \ref{['sec:discuss-4.1']}.
  • Figure 4: Large-scale environment of the CR3. The background shows the galaxy overdensity field at $z = 3.2\pm0.1$ based on COSMOS2025 LePhare photometric redshifts, with red (blue) indicating overdense (underdense) regions. Masked regions are shown in white.