DETECTION OF LYMAN-ALPHA EMISSION FROM A TRIPLY IMAGED z=6.85 GALAXY BEHIND MACS J2129.4-0741

We report the detection of Ly alpha emission at similar to 9538 angstrom 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 Ly alpha luminosity to be similar to 1.3 x 10(42) erg s(-1) and Ly alpha equivalent width to be 74 +/- 15 angstrom. Its intrinsic UV absolute magnitude at 1600 angstrom is -18.6 +/- 0.2 mag and stellar mass (1.5 +/- 0.3) x 10(7) M-circle dot, making it one of the faintest (intrinsic L-UV similar to 0.14 L-UV*) galaxies with Ly alpha detection at z similar to 7 to date. Its stellar mass is in the typical range for the galaxies thought to dominate the reionization photon budget at z greater than or similar to 7; the inferred Ly alpha escape fraction is high (greater than or similar to 10%), which could be common for sub-L* z greater than or similar to 7 galaxies with Ly alpha 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.