2016ApJ...823L..14H

We report the detection of Lyα emission at ∼9538 Å in the Keck/DEIMOS and Hubble Space Telescope WFC3 G102 grism data from a triply imaged galaxy at z=6.846±0.001 behind galaxy cluster MACS J2129.4-0741. Combining the emission line wavelength with broadband photometry, line ratio upper limits, and lens modeling, we rule out the scenario that this emission line is [O ii] at z=1.57. After accounting for magnification, we calculate the weighted average of the intrinsic Lyα luminosity to be ∼1.3×{10}^{{42} erg {s}}{-1} and Lyα equivalent width to be 74±15 Å. Its intrinsic UV absolute magnitude at 1600 Å is -18.6±0.2 mag and stellar mass (1.5±0.3)×{10}^{7}  {M}_☉ , making it one of the faintest (intrinsic L_UV∼0.14 L_UV^* ) galaxies with Lyα detection at z∼7 to date. Its stellar mass is in the typical range for the galaxies thought to dominate the reionization photon budget at z\gtrsim 7; the inferred Lyα escape fraction is high ( \gtrsim 10 %), which could be common for sub-L* z\gtrsim 7 galaxies with Lyα emission. This galaxy offers a glimpse of the galaxy population that is thought to drive reionization, and it shows that gravitational lensing is an important avenue for probing the sub-L* galaxy population.