Detection of Lyman-Alpha Emission From a Triple Imaged z=6.85 Galaxy Behind MACS J2129.4-0741

Abstract

We report the detection of Ly$\hat{I}\pm$ emission at $\sim 9538$Ã in the Keck/DEIMOS and \HST WFC3 G102 grism data from a triply-imaged galaxy at $z=6.846\pm 0.001$ 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 \oii at $z=1.57$. After accounting for magnification, we calculate the weighted average of the intrinsic Ly$\hat{I}\pm$ luminosity to be $\sim 1.3\times 10^{42}\mathrm{erg}{\mathrm{s}}^{-1}$ and Ly$\hat{I}\pm$ equivalent width to be $74\pm 15$Ã . Its intrinsic UV absolute magnitude at 1600Ã is $-18.6\pm 0.2$ mag and stellar mass $(1.5\pm 0.3)\times 10^7{M}_{\odot }$, making it one of the faintest (intrinsic $L_{UV}\sim 0.14L_{UV}^{\ast }$) galaxies with Ly$\hat{I}\pm$ detection at $z\sim 7$ 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 Ly$\hat{I}\pm$ escape fraction is high ($\gtrsim 10$\%), which could be common for sub-$L^{\ast }$ $z\gtrsim 7$ galaxies with Ly$\hat{I}\pm$ 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 to probe the sub-$L^{\ast }$ galaxy population.

Accepted by ApJ Letters