VANZELLA, ErosErosVANZELLADe Barros, S.S.De BarrosCUPANI, GuidoGuidoCUPANIKarman, W.W.KarmanGronke, M.M.GronkeBalestra, I.I.BalestraCoe, D.D.CoeMIGNOLI, MarcoMarcoMIGNOLIBrusa, M.M.BrusaCalura, F.F.CaluraCaminha, G. -B.G. -B.CaminhaCaputi, K.K.CaputiCASTELLANO, MARCOMARCOCASTELLANOChristensen, L.L.ChristensenCOMASTRI, AndreaAndreaCOMASTRICRISTIANI, StefanoStefanoCRISTIANIDijkstra, M.M.DijkstraFONTANA, AdrianoAdrianoFONTANAGiallongo, E.E.GiallongoGiavalisco, M.M.GiavaliscoGILLI, RobertoRobertoGILLIGRAZIAN, AndreaAndreaGRAZIANGrillo, C.C.GrilloKoekemoer, A.A.KoekemoerMENEGHETTI, MASSIMOMASSIMOMENEGHETTINONINO, MarioMarioNONINOPentericci, L.L.PentericciRosati, P.P.RosatiSchaerer, D.D.SchaererVerhamme, A.A.VerhammeVignali, C.C.VignaliZamorani, G.G.Zamorani2020-07-072020-07-0720162041-8205http://hdl.handle.net/20.500.12386/26353We present VLT/X-Shooter and MUSE spectroscopy of a faint F814W = 28.60 ± 0.33 ({M}<SUB>{UV</SUB>}=-17.0), low-mass (≲{10}<SUP>7</SUP>{M}<SUB>☉ </SUB>), and compact (R <SUB>eff</SUB> = 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 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<SUP>-1</SUP> and (de-lensed) fluxes spanning the interval 1.0× {10}<SUP>-19</SUP>{--}2× {10}<SUP>-18</SUP> erg s<SUP>-1</SUP> cm<SUP>-2</SUP> at signal-to-noise ratio (S/N) = 4-30. The double-peaked Lyα emission with {{∆ }}v({red}-{blue})=280(±7) km s<SUP>-1</SUP> and de-lensed fluxes {2.4}<SUB>({blue</SUB>)}| {8.5}<SUB>({red</SUB>)}× {10}<SUP>-18</SUP> erg s<SUP>-1</SUP> cm<SUP>-2</SUP> (S/N = {38}<SUB>({blue</SUB>)}| {110}<SUB>({red</SUB>)}) 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 \gt \quad 10 ratio. We detect C IV λ 1548,1550 resonant doublet in emission, each component with {FWHM}≲ 45 km s<SUP>-1</SUP> and redshifted by +51(±10) km s<SUP>-1</SUP> 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}<SUB>λ </SUB>={λ }<SUP>β </SUP>), consistent with a dust-free and young ≲20 Myr galaxy. Line ratios suggest an oxygen abundance 12 + log(O/H)\quad \lt \quad 7.8. We are witnessing an early episode of star formation in which a relatively low N <SUB>H I </SUB> 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. <P />Based on observations collected at the European Southern Observatory for Astronomical research in the Southern Hemisphere under ESO programs P095.A-0840, P095.A-0653, P186.A-0798.STAMPAenArticle10.3847/2041-8205/821/2/L272-s2.0-84964584382000374331900008https://iopscience.iop.org/article/10.3847/2041-8205/821/2/L272016ApJ...821L..27VFIS/05 - ASTRONOMIA E ASTROFISICA