The Magellan M2FS Spectroscopic Survey of High-$z$ Galaxies: Lyα Emitters at $z\approx6.6$ and the Evolution of Lyα Luminosity Function over $z\approx5.7-6.6$

Abstract

We present a sample of Lyα emitters (LAEs) at z ≈ 6.6 from our spectroscopic survey of highredshift galaxies using the multi-object spectrograph M2FS on the Magellan Clay telescope. The sample consists of 36 LAEs selected by the narrow-band (NB921) technique over nearly 2 deg 2 in the sky. These galaxies generally have high Lyα luminosities spanning a range of ∼3×10 42 −7×10 43 erg s −1 , and include some of the most Lyα-luminous galaxies known at this redshift. They show a positive correlation between the Lyα line width and Lyα luminosity, similar to the relation previously found in z ≈ 5.7 LAEs. Based on the spectroscopic sample, we calculate a sophisticated sample completeness correction and derive the Lyα luminosity function (LF) at z ≈ 6.6. We detect a density bump at the bright end of the Lyα LF that is significantly above the best-fit Schechter function, suggesting that very luminous galaxies tend to reside in overdense regions that have formed large ionized bubbles around them. By comparing with the z ≈ 5.7 Lyα LF, we confirm that there a rapid LF evolution at the faint end, but a lack of evolution at the bright end. The fraction of the neutral hydrogen in the intergalactic medium at z ≈ 6.6 estimated from such a rapid evolution is about ∼ 0.3 ± 0.1, supporting a rapid and rather late process of cosmic reionization.