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

TL;DR

This paper reports the discovery of a galaxy at redshift 3.19 with extremely low metallicity, providing evidence that Population III star formation may continue at cosmic noon, well after the early Universe.

Contribution

It presents the first detailed observational evidence of a nearly metal-free galaxy at high redshift, suggesting late Population III star formation.

Findings

Galaxy has extremely high Lyα and Hα equivalent widths.

Metallicity upper limit is below 7×10⁻³ Z☉, the lowest at cosmic noon.

Spectral modeling indicates a very young, low-mass stellar population.

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$\alpha$, H$\alpha$, and He I $\lambda$10830 emission. We measure rest-frame equivalent widths of EW$_0$(Ly$\alpha$) $=822\pm101$ Angstrom and EW$_0$(H$\alpha$) $=2814\pm327$ Angstrom, among the highest seen in star-forming systems. No metal lines, e.g. [O III] $\lambda\lambda4959,5007$, C IV $\lambda\lambda1548,1550$, have statistically significant detections, placing a 2-$\sigma$ 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-$\sigma$ upper limit is set to 12+log(O/H) < 6.95. The observed Ly$\alpha$/H$\alpha$ 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 ($\delta \approx -0.12$). 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 $z\gtrsim6$.