SEARCHING FORz∼ 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 COSMOS field. We have used two ultra-narrowband filters (filter width of ~8-9 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 10⁴ Mpc³ (total survey area ~760 arcmin²). 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.