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We report the LBT/MODS1 spectroscopic confirmation of two images of faintLyman alpha emitters at $z=6.4$ behind the Frontier Fields galaxy clusterMACSJ0717.5+3745. A wide range of lens models suggests that the two images arehighly magnified, with a strong lower limit of mu>5. These are the faintest z>6candidates spectroscopically confirmed to date. These may be also multipleimages of the same z=6.4 source as supported by their similar intrinsicproperties, 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 (restframeultraviolet) part of the spectral energy distributions and Ly-alpha lines arewell detected with S/N(m_1500)>~10 and S/N(Ly-alpha)~10-15. Adopting mu>5, theabsolute magnitudes, M_1500, and Ly-alpha fluxes, are fainter than -18.7 and2.8x10^(-18)erg/s/cm2, respectively. We find a very steep ultraviolet spectralslope beta=-3.0+/-0.5 (F_lambda=lambda^(beta)), implying that these are veryyoung, dust-free and low metallicity objects, made of standard stellarpopulations or even extremely metal poor stars (age<~30Myr, E(B-V)=0 andmetallicity 0.0-0.2 Z/Zsolar). The objects are compact (< 1 kpc^(2)), and witha stellar mass M* < 10^(8) M_solar. The very steep beta, the presence of theLy-alpha line and the intrinsic FWHM (<300 km/s) of these newborn objects donot exclude a possible leakage of ionizing radiation. We discuss thepossibility that such faint galaxies may resemble those responsible for cosmicreionization.

the astrophysical journal. letters

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