An extremely metal-poor star complex in the reionization era: Approaching Population III stars with JWST

Journal
Date Issued
2023
Author(s)
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Loiacono, F.
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Bergamini, P.
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Rosati, P.
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Grillo, C.
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Bradač, M.
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Adamo, A.
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Rihtaršič, G.
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Dickinson, M.
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Gronke, M.
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Willott, C.
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Messa, M.
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Sani, E.
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Acebron, A.
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Bolamperti, A.
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Caputi, K. I.
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Ricotti, M.
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Ravindranath, S.
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Strait, V.
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Martis, N.
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Pascale, R.
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Caminha, G. B.
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Annunziatella, M.
•
DOI
Abstract
We present JWST/Near Infrared Spectrograph (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 of ≲104 M⊙ and an absolute luminosity of MUV > −11.2 (mUV > 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. Although the stellar continuum is currently not detected in the Hubble and JWST/Near Infrared Camera (NIRCam) and Near Infrared Imager and Slitless Spectrograph (NIRISS) imaging, arclet-like shapes of Lyman and Balmer lines, Lyα, Hγ, Hβ and Hα are detected with NIRSpec IFS with signal-to-noise ratios (S/N) of approximately 5 − 13 and large equivalent widths (> 300 − 2000 Å), along with a remarkably weak [O III]λλ4959, 5007 at S/N ≃ 4. LAP1 shows a large ionizing photon production efficiency, log(ξion[erg Hz−1]) > 26. From the metallicity indexes R23 = ([O III] + [O II])/Hβ ≲ 0.74 and R3 = ([O III]/Hβ) = 0.55 ± 0.14, we derive an oxygen abundance of 12 + log(O/H)≲6.3. Intriguingly, the Hα emission is also measured in mirrored subcomponents where no [O III] 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). The formal stellar mass limit of the subcomponents would correspond to ∼103 M⊙ or MUV fainter than −10. Alternatively, this metal-free, pure line-emitting region could be the first case of a fluorescing H I 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.

Based on observations collected with the James Webb Space Telescope (JWST) and Hubble Space Telescope (HST). These observations are associated with JWST GO program n.1908 (PI: E. Vanzella) and GTO n.1208 (CANUCS, PI: C. Willot).

Volume
678
Start page
A173
Uri
Url
Issn Identifier
0004-6361
Ads BibCode
2023A&A...678A.173V
Rights
open.access
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