Searching for z ~ 7.7 Lyα 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 Cosmological Evolution Survey field. We have used two ultra-narrowband filters (filter width of ~8-9 A) on the NOAO Extremely Wide-Field Infrared Mosaic camera, installed on the Mayall 4 m 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 super(4) Mpc super(3) (total survey area ~760 arcmin super(2)). Each candidate has a line flux greater than 8 x 10 super(-18) erg s super(-1) cm super(-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.