2016ApJ...821L..27V

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 (R eff = 62 pc) freshly star-forming galaxy at z = 3.1169 magnified (16x) 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 λ1548,1550, He ii λ1640, O iii]λ1661,1666, C iii]λλ1907,1909, Hβ, [O iii]λλ4959,5007 emission lines with FWHM\lesssim 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 Δv(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]λ5007/ [O ii]λ3727\gt   10 ratio. We detect C iv λ1548,1550 resonant doublet in emission, each component with FWHM\lesssim 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 λ1548,1550 and He ii λ1640 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)  \lt   7.8. 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.