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

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 $z=3.193\pm 0.016$. 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$) $=822\pm 101$ Angstrom and EW${}_0$(H$\alpha$) $=2814\pm 327$ Angstrom, among the highest seen in star-forming systems. No metal lines, e.g. [O III] $\lambda \lambda 4959,5007$, C IV $\lambda \lambda 1548,1550$, have statistically significant detections, placing a 2-$\sigma$ upper limit on the gas-phase metallicity of 12+log(O/H) < 6.52 ($Z<7\times 10^{-3}{Z}_{\odot }$) 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 $13.9\pm 2.5$, indicating negligible dust attenuation. Spectral energy distribution modeling with Pop III stellar templates indicates a very young ($\sim 2$ Myr), low-mass ($M_{\ast }\approx 6.1\times 10^5{M}_{\odot }$) 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 $z\gtrsim 6$.

12 pages, 4 figures, 2 tables. Accepted version for ApJL. Comments welcome!

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