Searching for z~7.7 Lyman Alpha Emitters in the COSMOS Field with
  NEWFIRM
  The study of Ly-alpha emission in the high-redshift universe is a useful
probe of the epoch of reionization, as the Ly-alpha line should be attenuated
by the intergalactic medium (IGM) at low to moderate neutral hydrogen
fractions. Here we present the results of a deep and wide imaging search for
Ly-alpha emitters in the COSMOS field. We have used two ultra-narrowband
filters (filter width of ~8-9 {\deg}A) on the NEWFIRM camera, installed on the
Mayall 4m telescope at Kitt Peak National Observatory, in order to isolate
Ly-alpha emitters at z = 7.7; such ultra-narrowband imaging searches have
proved to be excellent at detecting Ly-alpha emitters. We found 5-sigma
detections of four candidate Ly-alpha emitters in a survey volume of 2.8 x 10^4
Mpc^3 (total survey area ~760 arcmin^2). Each candidate has a line flux greater
than 8 x 10^-18 erg s^-1 cm^-2. Using these results to construct a luminosity
function and comparing to previously established Ly-alpha luminosity functions
at z = 5.7 and z = 6.5, we find no conclusive evidence for evolution of the
luminosity function between z = 5.7 and z = 7.7. Statistical Monte Carlo
simulations suggest that half of these candidates are real z = 7.7 targets, and
spectroscopic follow-up will be required to verify the redshift of these
candidates. However, our results are consistent with no strong evolution in the
neutral hydrogen fraction of the IGM between z = 5.7 and z = 7.7, even if only
one or two of the z = 7.7 candidates are spectroscopically confirmed.
