Letter to the Editor

A pristine, star-forming complex at z = 4.19^★

¹ INAF – OAS, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna via Gobetti 93/3 I-40129 Bologna, Italy
² Max-Planck-Institut für Astrophysik Karl-Schwarzschild-Str. 1 D-85748 Garching, Germany
³ INAF – Osservatorio Astronomico di Roma, Via Frascati 33 00078 Monteporzio Catone Rome, Italy
⁴ Department of Physics & Astronomy, University College London London WC1E 6BT, UK
⁵ Department of Astronomy, Oskar Klein Centre, Stockholm University, AlbaNova University Centre SE-106 91 Stockholm, Sweden
⁶ Department of Physics and Astronomy, University of California, Los Angeles 430 Portola Plaza Los Angeles CA 90095, USA
⁷ Dipartimento di Fisica, Università degli Studi di Milano Via Celoria 16 I-20133 Milano, Italy
⁸ INAF – IASF Milano via A. Corti 12 I-20133 Milano, Italy
⁹ Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Ferrara Via Saragat 1 I-44122 Ferrara, Italy

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 8 September 2025
Accepted: 21 December 2025

Abstract

We report the discovery of a faint (M₁₇₀₀ ≃ −12.2), oxygen-deficient, strongly lensed ionizing source – dubbed Lensed And Pristine 2 (LAP2) – at a spectroscopic redshift of z = 4.19. LAP2 appears to be isolated and lies very close to the caustic produced by the lensing galaxy cluster Abell 2744. It was observed with the James Webb Space Telescope (JWST) NIRSpec MSA in prism mode as part of the UNCOVER program. The NIRSpec spectrum reveals prominent Lyα (7.1σ), clear Hα (6.2σ), tentative Hβ (≃2.8σ) emissions and no detectable [O/III]λλ4959,5007 (∼7 times fainter than Hα). The inferred [O III] 2σ upper limit corresponds to an R3 index < 0.85 (assuming the Hα/Hβ = 2.86 case B recombination ratio), which, under high-ionization conditions, implies a metallicity of Z < 6 × 10⁻³ Z_⊙. The combination of faint ultraviolet luminosity, a large rest-frame Hα equivalent width (≃650 Å), and an extremely compact size (< 10 pc) suggests that LAP2 is being caught in an early, pristine formation phase consistent with an instantaneous-burst scenario, with an estimated stellar mass of at most a few ×10⁴ M_⊙. Deep VLT/MUSE observations further reveal copious Lyα emission forming an arclet that straddles the critical line. LAP2 joins the rare class of extremely metal-poor star-forming complexes that the JWST has started to unveil at redshifts 3–7, and it provides a glimpse into a still very poorly explored low-luminosity regime.