HIGH-RESOLUTION SPECTROSCOPY OF A YOUNG, LOW-METALLICITY OPTICALLY THIN L = 0.02L* STAR-FORMING GALAXY AT z = 3.12

We present VLT/X-Shooter and MUSE spectroscopy of a faint F814W = 28.60 ± 0.33 ( M UV = − 17.0 ), low-mass (≲ 10 7 M ⊙ ), and compact (Reff = 62 pc) freshly star-forming galaxy at z = 3.1169 magnified (16×) by the Hubble Frontier Fields galaxy cluster Abell S1063. Gravitational lensing allows for a significant jump toward low-luminosity regimes, in moderately high-resolution spectroscopy ( R = λ / d λ ∼ 3000 – 7400 ). We measured C iv λ 1548 , 1550 , He ii λ 1640 , O iii] λ 1661 , 1666 , C iii] λ λ 1907 , 1909 , Hβ, [O iii] λ λ 4959 , 5007 emission lines with FWHM ≲ 50 km s−1 and (de-lensed) fluxes spanning the interval 1.0 × 10 − 19 – 2 × 10 − 18 erg s−1 cm−2 at signal-to-noise ratio (S/N) = 4–30. The double-peaked Lyα emission with Δ v ( red − blue ) = 280 (±7) km s−1 and de-lensed fluxes 2.4 ( blue ) ∣ 8.5 ( red ) × 10 − 18 erg s−1 cm−2 (S/N = 38 ( blue ) ∣ 110 ( red ) ) indicate a low column density of neutral hydrogen gas consistent with a highly ionized interstellar medium as also inferred from the large [O iii] λ 5007 / [O ii] λ 3727 > 10 ratio. We detect C iv λ 1548 , 1550 resonant doublet in emission, each component with FWHM ≲ 45 km s−1 and redshifted by +51(±10) km s−1 relative to the systemic redshift. We interpret this as nebular emission tracing an expanding optically thin interstellar medium. Both C iv λ 1548 , 1550 and He ii λ 1640 suggest the presence of hot and massive stars (with a possible faint active galactic nucleus). The ultraviolet slope is remarkably blue, β = − 2.95 ± 0.20 ( F λ = λ β ), consistent with a dust-free and young ≲20 Myr galaxy. Line ratios suggest an oxygen abundance 12 + log(O/H) < 7.8 . We are witnessing an early episode of star formation in which a relatively low NH i and negligible dust attenuation might favor a leakage of ionizing radiation. This galaxy currently represents a unique low-luminosity reference object for future studies of the reionization epoch with the James Webb Space Telescope.