THE LUMINOSITY FUNCTION OF Lyα EMITTERS AT REDSHIFT z = 7.7

Ly alpha emission lines should be attenuated in a neutral intergalactic medium. 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 Ly alpha emitting galaxies using an ultra-narrowband filter (filter width = 9 (A) over circle) 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 6 x 10(-18) erg cm(-2) s(-1) (5 sigma in 2 '' aperture). We also performed a detailed Monte Carlo simulation incorporating the instrumental effects to estimate the expected number of Ly alpha emitters in our survey and found that we should expect to detect one Ly alpha emitter, assuming a non-evolving Ly alpha luminosity function (LF) between z = 6.5 and z = 7.7. Even if one of the present candidates is spectroscopically confirmed as a z approximate to 8 Ly alpha emitter, it would indicate that there is no significant evolution of the Ly alpha LF from z = 3.1 to z approximate to 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 mu m) narrowband searches using custom filters designed to avoid the OH emission lines that make up most of the sky background.