The Magellan M2FS Spectroscopic Survey of High-z Galaxies: Lyα Emitters at z ≈ 6.6 and the Evolution of Lyα Luminosity Function over z ≈ 5.7–6.6

Abstract

We present a sample of Ly α emitters (LAEs) at z ≈ 6.6 from our spectroscopic survey of high-redshift galaxies using the multi-object spectrograph M2FS on the Magellan Clay telescope. The sample consists of 36 LAEs selected by the narrowband (NB921) technique over nearly 2 deg 2 in the sky. These galaxies generally have high Ly α luminosities spanning a range of ∼3 × 10 42 –7 × 10 43 erg s −1 , and include some of the most Ly α -luminous galaxies known at this redshift. They show a positive correlation between the Ly α line width and Ly α 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 Ly α luminosity function (LF) at z ≈ 6.6. We detect a density bump at the bright end of the Ly α 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 Ly α LF, we confirm that there is 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 an evolution is about 0.3 ± 0.1, supporting a rapid and rather late process of cosmic reionization.