DETECTION of LYMAN-ALPHA EMISSION from A TRIPLY IMAGED z = 6.85 GALAXY behind MACS J2129.4-0741

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 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 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 , making it one of the faintest (intrinsic ) galaxies with Lyα detection at to date. Its stellar mass is in the typical range for the galaxies thought to dominate the reionization photon budget at the inferred Lyα escape fraction is high (%), which could be common for sub-L∗ 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