First Results from the Lyman Alpha Galaxies in the Epoch of Reionization (LAGER) Survey: Cosmological Reionization at z ~ 7

We present the first results from the ongoing LAGER project (Lyman Alpha Galaxies in the Epoch of Reionization), which is the largest narrowband survey for $z\sim$ 7 galaxies to date. Using a specially built narrowband filter NB964 for the superb large-area Dark-Energy Camera (DECam) on the NOAO/CTIO 4m Blanco telescope, LAGER has collected 34 hours NB964 narrowband imaging data in the 3 deg${}^2$ COSMOS field. We have identified 23 Lyman Alpha Emitter (LAE) candidates at $z$ = 6.9 in the central 2-deg${}^2$ region, where DECam and public COSMOS multi-band images exist. The resulting luminosity function can be described as a Schechter function modified by a significant excess at the bright end (4 galaxies with $L_{Ly\alpha }\sim$ 10${}^{43.4\pm 0.2}$ erg s${}^{-1}$). The number density at $L_{Ly\alpha }\sim$ 10${}^{43.4\pm 0.2}$ erg s${}^{-1}$ is little changed from z= 6.6, while at fainter $L_{Ly\alpha }$ it is substantially reduced. Overall, we see a fourfold reduction in Ly$\alpha$ luminosity density from $z$ = 5.7 to 6.9. Combined with a more modest evolution of the continuum UV luminosity density, this suggests a factor of $\sim 3$ suppression of Ly$\alpha$ by radiative transfer through the $z\sim$ 7 intergalactic medium (IGM). It indicates an IGM neutral fraction $x_{HI}$ $\sim$ 0.4–0.6 (assuming Ly$\alpha$ velocity offsets of 100-200 km s${}^{-1}$). The changing shape of the Ly$\alpha$ luminosity function between $z\lesssim 6.6$and$z=6.9$ supports the hypothesis of ionized bubbles in a patchy reionization at $z\sim$ 7.