Abstract
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
$5$ 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. We spectroscopically confirm the two most
luminous Ly$\alpha$ emitters ever found at $z=6.604$ and $6.541$ in the COSMOS
field using Keck/DEIMOS and VLT/FORS2. 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
Himiko-like sources are not as rare as previously thought, with number
densities of $1.5\times10^-5$ 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 observed faint and bright
LAEs. This differential evolution was not addressed in previous studies, or
discarded as cosmic variance, but we argue instead 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.
Description
[1502.07355] Identification of the brightest Ly\alpha\ emitters at z=6.6: implications for the evolution of the luminosity function in the re-ionisation era
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