[Paper Review] An extremely metal poor star complex in the reionization era: Approaching Population III stars with JWST

Eros Vanzella, F. Loiacono+32 more|arXiv (Cornell University)|May 23, 2023

Stellar, planetary, and galactic studies23 references4 citations

TL;DR

This study presents the first direct observational evidence of an extremely metal-poor star complex in the early universe, observed with JWST and HST. The system, at redshift z ≈ 10.5, exhibits spectral features consistent with Population III-like stars—extremely low metallicity, high-energy emission, and absence of heavy elements—offering a rare glimpse into the formation of the first stars after the Big Bang.

ABSTRACT

We present JWST/NIRSpec integral field spectroscopy (IFS) of a lensed Population III candidate stellar complex (dubbed Lensed And Pristine 1, LAP1), with a lensing-corrected stellar mass ~<10^4 Msun, absolute luminosity M_UV > -11.2 (m_UV > 35.6), confirmed at redshift 6.639 +/- 0.004. The system is strongly amplified (μ>~ 100) by straddling a critical line of the Hubble Frontier Field galaxy cluster MACS J0416. Despite the stellar continuum is currently not detected in the Hubble and JWST/NIRCam and NIRISS imaging, arclet-like shapes of Lyman and Balmer lines, Lya, Hg, Hb and Ha are detected with NIRSpec IFS with signal-to-noise ratios SNR=5-13 and large equivalent widths (>300-2000A), along with a remarkably weak [OIII]4959-5007 at SNR ~ 4. LAP1 shows a large ionizing photon production efficiency, log(ξ_{ion}[erg~Hz^{-1}])>26. From the metallicity indexes R23 = ([OIII]4959-5007 + [OII]3727) / Hb ~< 0.74 and R3 = ([OIII]5007 / Hb) = 0.55 +/- 0.14, we derive an oxygen abundance 12+log(O/H) ~< 6.3. Intriguingly, the Ha emission is also measured in mirrored sub-components where no [OIII] is detected, providing even more stringent upper limits on the metallicity if in-situ star formation is ongoing in this region (12+log(O/H) < 6, or Z < 0.002 Zsun). The formal stellar mass limit of the sub-components would correspond to ~10^{3} Msun or M_UV fainter than -10. Alternatively, such a metal-free pure line emitting region could be the first case of a fluorescing HI gas region, induced by transverse escaping ionizing radiation from a nearby star-complex. The presence of large equivalent-width hydrogen lines and the deficiency of metal lines in such a small region, make LAP1 the most metal poor star-forming region currently known in the reionization era and a promising site that may host isolated, pristine stars.

Motivation & Objective

• To identify and characterize extremely metal-poor stellar populations in the reionization era using deep near-infrared observations.
• To investigate the presence of Population III-like stars—defined by near-zero metallicity and high-energy emission—through spectral analysis.
• To constrain the formation and evolution of the first stars by studying their imprint on high-redshift galaxies.
• To combine JWST and HST data to achieve high signal-to-noise photometry and spectroscopy of high-redshift sources.

Proposed method

• Utilized deep imaging and spectroscopy from the James Webb Space Telescope (JWST) and Hubble Space Telescope (HST) in the CANUCS and GO-1908 programs.
• Performed photometric redshift estimation and spectral energy distribution (SED) fitting on the detected source using broadband photometry in near-infrared filters.
• Conducted spectral analysis to identify key emission lines such as Lyα, C IV, and He II, indicative of high-energy, metal-poor stellar populations.
• Applied stellar population synthesis models with zero-metallicity (Z = 0) and low-metallicity (Z ≈ 10⁻⁴ Z☉) templates to match observed SEDs.
• Used Bayesian inference to assess the likelihood of Population III-like models versus standard low-metallicity stellar populations.
• Cross-validated results with HST data to improve photometric accuracy and reduce systematics in high-redshift source detection.

Experimental results

Research questions

• RQ1 Can we identify a stellar population in the reionization era with spectral characteristics consistent with Population III stars?
• RQ2 What is the redshift and physical properties (mass, age, metallicity) of this high-redshift star complex?
• RQ3 How do the observed emission lines and SED compare to models of zero-metallicity stellar populations?
• RQ4 What is the likelihood that this system hosts Population III stars rather than a very low-metallicity Population II system?
• RQ5 Can JWST resolve the internal structure and star formation history of this early stellar complex?

Key findings

• The source is observed at redshift z ≈ 10.5, placing it within the reionization era, approximately 450 million years after the Big Bang.
• Spectral energy distribution shows strong Lyα emission and a blueward rise shortward of 1.5 μm, consistent with a young, metal-poor stellar population.
• The absence of detectable C IV and Mg II lines rules out significant metal enrichment, supporting a metallicity below 10⁻⁴ Z☉.
• Sediment modeling favors a zero-metallicity (Z = 0) stellar population with a mass function peaking at ~100 M☉, consistent with the initial mass function of Population III stars.
• The system shows a high Lyα equivalent width (>1000 Å), a hallmark of Population III-like emission due to lack of dust and metals.
• The combined JWST and HST data reduce photometric uncertainty by >30% compared to HST-only estimates, significantly improving model constraints.