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$^{43.4\pm0.2}$ erg
s$^{-1}$). The number density at $L_{Ly\alpha}\sim$ 10$^{43.4\pm0.2}$ 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.
