A Metal-free Galaxy at <i>z</i> = 3.19? Evidence of Late Population III Star Formation at Cosmic Noon
Sijia Cai, Mingyu Li, Zheng Cai, Yunjing Wu, F. Yu, Mark Dickinson, Fengwu Sun, Xiaohui Fan, Ben Wang, Fergus Cullen, Fuyan Bian, Xiaojing Lin, Jiaqi Zou
Abstract
Abstract Star formation from metal-free gas, the hallmark of the first generation of Population III (Pop III) stars, was long assumed to occur only in the very early Universe. We report the discovery of Metal-Pristine Galaxy COSMOS Redshift 3 (MPG-CR3, hereafter CR3), an extremely metal-poor galaxy at redshift z = 3.193 ± 0.016. From JWST, Very Large Telescope, and Subaru observations, CR3 exhibits exceptionally strong Ly α , H α , and He i λ 10830 emissions. We measure rest-frame equivalent widths of EW 0 (Ly α ) = 822 ± 101 Å and EW 0 (H α ) = 2814 ± 327 Å, among the highest seen in star-forming systems. No metal lines, e.g., [O iii ] λλ 4959, 5007, C iv λλ 1548, 1550, have statistically significant detections, placing a 2 σ upper limit on the gas-phase metallicity of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>12</mml:mn> <mml:mo>+</mml:mo> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">O</mml:mi> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">H</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo><</mml:mo> <mml:mn>6.52</mml:mn> </mml:math> ( Z < 7 × 10 −3 Z ⊙ ) with strong-line calibration established by JWST, making it the most metal-poor galaxy known at cosmic noon. Considering systematic uncertainties of ≳0.3 dex in the calibrations, the most conservative 2 σ upper limit is set to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>12</mml:mn> <mml:mo>+</mml:mo> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">O</mml:mi> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">H</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo><</mml:mo> <mml:mn>6.95</mml:mn> </mml:math> . The observed Ly α /H α flux ratio is 13.9 ± 2.5, indicating negligible dust attenuation. Spectral energy distribution modeling with Pop III stellar templates indicates a very young (∼2 Myr), low-mass ( M * ≈ 6.1 × 10 5 M ⊙ ) stellar population. Further, the photometric redshifts reveal that CR3 could reside in a slightly underdense environment ( δ ≈ −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 ≳ 6.