2026A&A...705L..12V

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.