CHARACTERIZING FAINT GALAXIES IN THE REIONIZATION EPOCH: LBT CONFIRMS TWO L < 0.2 L☆ 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 Ly alpha 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 mu > 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 Ly alpha lines are well detected with S/N(m(1500)) greater than or similar to 10 and S/ N(Ly alpha) similar or equal to 10-15. Adopting mu > 5, the absolutemagnitudes, M-1500, and Lya fluxes are fainter than-18.7 and 2.8 x 10(-18) erg s(-1) cm(-2), respectively. We find a very steep ultraviolet spectral slope beta = -3.0 +/- 0.5 (F-lambda = lambda(beta)), implying that these are very young, dust-free, and low metallicity objects, made of standard stellar populations or even extremely metal poor stars (age less than or similar to 30 Myr, E(B - V) = 0 and metallicity 0.0-0.2 Z/Z(circle dot)). The objects are compact (<1 kpc(2)) and with a stellar mass M-star < 10(8) M-circle dot. The very steep beta, the presence of the Ly alpha line, and the intrinsic FWHM (<300 km s(-1)) of these newborn objects do not exclude a possible leakage of ionizing radiation. We discuss the possibility that such faint galaxies may resemble those responsible for cosmic reionization.