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 an faint F814W=28.60+/-0.33
(Muv=-17.0), low mass (~<10^7 Msun) and compact (Reff=62pc) 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=lambda/dlambda ~ 3000-7400). We measured CIV1548,1550, HeII1640,
OIII]1661,1666, CIII]1907,1909, Hbeta, [OIII]4959,5007, emission lines with
FWHM< 50 km/s and (de-lensed) fluxes spanning the interval 1.0x10^-19 -
2.0x10^-18 erg/s/cm2 at S/N=4-30. The double peaked Lya emission with
Delta_v(red-blue) = 280(+/-7)km/s and de-lensed fluxes
2.4_(blue)|8.5_(red)x10^-18 erg/s/cm2 (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 [OIII]5007/[OII]3727>10
ratio. We detect CIV1548,1550 resonant doublet in emission, each component with
FWHM ~< 45 km/s, and redshifted by +51(+/-10)km/s relative to the systemic
redshift. We interpret this as nebular emission tracing an expanding
optically-thin interstellar medium. Both CIV1548,1550 and HeII1640 suggest the
presence of hot and massive stars (with a possible faint AGN). The ultraviolet
slope is remarkably blue, beta =-2.95 +/- 0.20 (F_lambda=lambda^beta),
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 NHI 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 JWST.
