Identification of the brightest Ly\alpha\ emitters at z=6.6:
  implications for the evolution of the luminosity function in the
  re-ionisation era
  Using wide field narrow-band surveys, we provide a new measurement of the
$z=6.6$ Lyman-$\alpha$ Emitter (LAE) luminosity function (LF), which
constraints the bright end for the first time. We use a combination of archival
narrow-band NB921 data in UDS and new NB921 measurements in SA22 and
COSMOS/UltraVISTA, all observed with the Subaru telescope, with a total area of
$\sim 5$ deg$^2$. We exclude lower redshift interlopers by using broad-band
optical and near-infrared photometry and also exclude three supernovae with
data split over multiple epochs. Combining the UDS and COSMOS samples we find
no evolution of the bright end of the Ly$\alpha$ LF between $z=5.7$ and $6.6$,
which is supported by spectroscopic follow-up, and conclude that sources with
\emph{Himiko}-like luminosity are not as rare as previously thought, with
number densities of $\sim 1.5\times10^{-5}$ Mpc$^{-3}$. Combined with our
wide-field SA22 measurements, our results indicate a non-Schechter-like bright
end of the LF at $z=6.6$ and a different evolution of \emph{observed} faint and
bright LAEs. This differential evolution is also seen in the spectroscopic
follow-up of UV selected galaxies and is now also confirmed for Ly$\alpha$
emitters, and we argue that it may be an effect of re-ionisation. Using a
toy-model, we show that such differential evolution of the LF is expected,
since brighter sources are able to ionise their surroundings earlier, such that
Ly$\alpha$ photons are able to escape. Our targets are excellent candidates for
detailed follow-up studies and provide the possibility to give a unique view on
the earliest stages in the formation of galaxies and re-ionisation process.
