A Pristine Star-Forming Complex at z=4.19

Abstract

We report the discovery of a faint (M_1700 ~ -12.2), oxygen-deficient strongly-lensed ionizing source -- dubbed LAP2 (Lensed And Pristine 2) -- 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 Lya emission (7.1 sigma), clear Ha emission (6.2 sigma), tentative Hb emission (2.8 sigma), and no detectable [OIII]4959,5007 (~ 7 times fainter than Ha). The inferred [OIII] 2 sigma upper limit corresponds to an R3 index <0.85 (assuming the Ha/Hb ~ 2.86 case~B recombination ratio), which, under high-ionization conditions, implies a metallicity of Z < 6 X 10^(-3) Z_sun. The combination of faint ultraviolet luminosity, large rest-frame Ha equivalent width (~ 650 A), and 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 10^4 Msun. Deep VLT/MUSE observations further reveal copious Lya emission forming an arclet that straddles the critical line. LAP2 thus joins the rare class of extremely metal-poor star-forming complexes that JWST has started to unveil at redshifts 3 - 7, and provides a rare glimpse into a still very poorly explored faint-luminosity regime.

4 pages, 3 figures, 1 table. A&A accepted

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