CHARACTERIZING FAINT GALAXIES IN THE REIONIZATION EPOCH: LBT CONFIRMS TWO L< 0.2L* SOURCES AT z= 6.4 BEHIND THE CLASH/FRONTIER FIELDS CLUSTER MACS0717.5+3745

We report the LBT/MODS1 spectroscopic confirmation of two images of faint Lyα emitters at z = 6.4 behind the Frontier Fields galaxy cluster MACSJ0717.5+3745. A wide range of lens models suggests that the two images are highly magnified, with a strong lower limit of μ > 5. These are the faintest z > 6 candidates spectroscopically confirmed to date. These may also be multiple images of the same z = 6.4 source as supported by their similar intrinsic properties, but the lens models are inconclusive regarding this interpretation. To be cautious, we derive the physical properties of each image individually. Thanks to the high magnification, the observed near-infrared (restframe ultraviolet) part of the spectral energy distributions and Lyα lines are well detected with S/N(m 1500) >~ 10 and S/N(Lyα) ~= 10-15. Adopting μ > 5, the absolute magnitudes, M 1500, and Lyα fluxes are fainter than -18.7 and 2.8 × 10-18 erg s-1 cm-2, respectively. We find a very steep ultraviolet spectral slope β = -3.0 ± 0.5 (F λ = λβ), implying that these are very young, dust-free, and low metallicity objects, made of standard stellar populations or even extremely metal poor stars (age <~ 30 Myr, E(B - V) = 0 and metallicity 0.0-0.2 Z/Z ⊙). The objects are compact (<1 kpc2) and with a stellar mass M sstarf < 108 M ⊙. The very steep β, the presence of the Lyα line, and the intrinsic FWHM (