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 (MUV = -17.0), low-mass (≲107M⊙), and compact (R eff = 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/dλ ∼ ∼ 3000-7400). We measured C iv , He ii , O iii], C iii], Hβ, [O iii] emission lines with km s-1 and (de-lensed) fluxes spanning the interval erg s-1 cm-2 at signal-to-noise ratio (S/N) = 4-30. The double-peaked Lyα emission with (±7) km s-1 and de-lensed fluxes erg s-1 cm-2 (S/N = ) indicate a low column density of neutral hydrogen gas consistent with a highly ionized interstellar medium as also inferred from the large [O iii]/ [O ii] ratio. We detect C iv resonant doublet in emission, each component with 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 and He ii 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 (Fλ λβ), consistent with a dust-free and young ≲20 Myr galaxy. Line ratios suggest an oxygen abundance 12 + log(O/H). We are witnessing an early episode of star formation in which a relatively low N H 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.