Abstract
We present a new measurement of the Ly$\alpha$ luminosity function at
redshift $z=6.9$, finding moderate evolution from $z=5.7$ that is consistent
with a fully or largely ionized $z\sim7$ intergalactic medium. Our result is
based on four fields of the LAGER (Lyman Alpha Galaxies in the Epoch of
Reionization) project. Our survey volume of $6.1\times10^6$ Mpc$^3$ is
double that of the next largest $z7$ survey. We combine two new LAGER
fields (WIDE12 and GAMA15A) with two previously reported LAGER fields (COSMOS
and CDFS). In the new fields, we identify $N=95$ new $z=6.9$ Ly$\alpha$
emitters (LAEs); characterize our survey's completeness and reliability; and
compute Ly$\alpha$ luminosity functions. The best-fit Schechter luminosity
function parameters for all four LAGER fields are in good general agreement.
Two fields (COSMOS and WIDE12) show evidence for a bright-end excess above the
Schechter function fit. We find that the Ly$\alpha$ luminosity density declines
at the same rate as the UV continuum LF from $z=5.7$ to $z=6.9$. This is
consistent with an intergalactic medium that was fully ionized as early as
redshift $z7$, or with a volume-averaged neutral hydrogen fraction of
$x_HI < 0.33$ at $1\sigma$.
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