Deep LBT/LUCI spectroscopy of an Lyα emitter candidate at z ≃ 7.7

We present deep spectroscopic observations of an Lyα emitter (LAE) candidate at z ≃ 7.7 using the infrared spectrograph LUCI on the 2 × 8.4 m Large Binocular Telescope (LBT). The candidate is the brightest among the four z ≃ 7.7 LAE candidates found in a narrowband imaging survey by Krug et al. Our spectroscopic data include a total of 7.5 hr of integration with LBT/LUCI and are deep enough to significantly (3.2σ-4.9σ) detect the Lyα emission line of this candidate based on its Lyα flux 1.2 × 10^-17 erg s^-1 cm^-2 estimated from the narrowband photometry. However, we do not find any convincing signal at the expected position of its Lyα emission line, suggesting that this source is not an LAE at z ≃ 7.7. The non-detection in this work, together with the previous studies of z ≃ 7.7 LAEs, puts a strong constraint on the bright-end Lyα luminosity function (LF) at z ≃ 7.7. We find a rapid evolution of the Lyα LF from z ≃ 6.5 to 7.7: the upper limit of the z ≃ 7.7 LF is more than five times lower than the z ≃ 6.5 LF at the bright end (f≥ 1.0 × 10^-17 erg s^-1 cm ^-2 or L≥ 6.9 × 10⁴² erg s^-1). This is likely caused by an increasing neutral fraction in the intergalactic medium that substantially attenuates Lyα emission at z ≃ 7.7.