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

¹ INAF – OAS, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
e-mail: eros.vanzella@inaf.it
² Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy
³ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monteporzio Catone, Rome, Italy
⁴ Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Ferrara, Via Saragat 1, 44122 Ferrara, Italy
⁵ INAF – IASF Milano, Via A. Corti 12, 20133 Milano, Italy
⁶ University of Ljubljana, Department of Mathematics and Physics, Jadranska ulica 19, 1000 Ljubljana, Slovenia
⁷ Department of Physics and Astronomy, University of California Davis, 1 Shields Avenue, Davis, CA 95616, USA
⁸ Department of Astronomy, Oskar Klein Centre, Stockholm University, AlbaNova University Centre, 106 91 Stockholm, Sweden
⁹ NSF’s National Optical-Infrared Astronomy Research Laboratory, 950 N. Cherry Ave., Tucson, AZ 85719, USA
¹⁰ Max Planck Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85748 Garching bei München, Germany
¹¹ INAF Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹² NRC Herzberg, 5071 West Saanich Rd, Victoria, BC V9E 2E7, Canada
¹³ Département d’Astronomie, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
¹⁴ European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Santiago 19001, Chile
¹⁵ Dipartimento di Fisica e Astronomia, Università degli Studi di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹⁶ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
¹⁷ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands
¹⁸ Department of Astronomy, University of Maryland, College Park 20742, USA
¹⁹ Space Telescope Science Institute (STScI), 3700 San Martin Drive, Baltimore, MD 21218, USA
²⁰ Dipartimento di Fisica “E.R. Caianiello”, Università Degli Studi di Salerno, Via Giovanni Paolo II, 84084 Fisciano, SA, Italy
²¹ INAF – Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Napoli, Italy
²² Cosmic Dawn Center (DAWN), Copenhagen, Denmark
²³ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen N, Denmark
²⁴ Institute for Computational Astrophysics and Department of Astronomy & Physics, Saint Mary’s University, 923 Robie Street, Halifax, NS B3H 3C3, Canada
²⁵ Technical University of Munich, Department of Physics, James-Franck-Straße 1, 85748 Garching, Germany
²⁶ Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir km 4, Torrejòn de Ardoz, 28850 Madrid, Spain
²⁷ INAF-Trieste Astronomical Observatory, Via Bazzoni 2, 34124 Trieste, Italy

Received: 23 May 2023
Accepted: 28 July 2023

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 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. 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(ξ_ion[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 M_UV 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.