The luminosity function of Lyα emitters at redshift z = 7.7

Lyα emission lines should be attenuated in a neutral intergalactic medium. Therefore, the visibility of Lyα 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 Lyα emitting galaxies using an ultra-narrowband filter (filter width = 9Å) on the NEWFIRM imager at the Kitt Peak National Observatory. We found four candidate Lyα emitters in a survey volume of 1.4 × 10⁴ Mpc³, with a line flux brighter than 6 × 10^-18 erg cm^-2 s^-1 (5σ in 2″ aperture). We also performed a detailed Monte Carlo simulation incorporating the instrumental effects to estimate the expected number of Lyα emitters in our survey and found that we should expect to detect one Lyα emitter, assuming a non-evolving Lyα 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 Lyα emitter, it would indicate that there is no significant evolution of the Lyα 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 (1.06μm) narrowband searches using custom filters designed to avoid the OH emission lines that make up most of the sky background.