The Luminosity Function of Lyman alpha Emitters at Redshift z=7.7
  Lyman alpha (Lya) emission lines should be attenuated in a neutral
intergalactic medium (IGM). Therefore the visibility of Lya emitters at high
redshifts can serve as a valuable probe of reionization at about the 50% level.
We present an imaging search for z=7.7 Lya emitting galaxies using an
ultra-narrowband filter (filter width= 9A) on the NEWFIRM imager at the Kitt
Peak National Observatory. We found four candidate Lya emitters in a survey
volume of 1.4 x 10^4 Mpc^3, with a line flux brighter than 6x10^-18 erg/cm^2/s
(5 sigma in 2" aperture). We also performed a detailed Monte-Carlo simulation
incorporating the instrumental effects to estimate the expected number of Lya
emitters in our survey, and found that we should expect to detect one Lya
emitter, assuming a non-evolving Lya luminosity function (LF) between z=6.5 and
z=7.7. Even if one of the present candidates is spectroscopically confirmed as
a z~8 Lya emitter, it would indicate that there is no significant evolution of
the Lya LF from z=3.1 to z~8. While firm conclusions would need both
spectroscopic confirmations and larger surveys to boost the number counts of
galaxies, we successfully demonstrate the feasibility of sensitive
near-infrared (1.06 um) narrow-band searches using custom filters designed to
avoid the OH emission lines that make up most of the sky background.
