The Magellan M2FS Spectroscopic Survey of High-Redshift Galaxies: A
  Sample of 260 Ly$\alpha$ Emitters at Redshift $z\approx5.7$
  We present a spectroscopic survey of Ly$\alpha$ emitters (LAEs) at
$z\approx5.7$ using the multi-object spectrograph M2FS on the Magellan Clay
telescope. This is part of a high-redshift galaxy survey carried out in several
well-studied deep fields. These fields have deep images in multiple UV/optical
bands, including a narrow NB816 band that has allowed an efficient selection of
LAE candidates at $z\approx5.7$. Our sample consists of 260 LAEs and covers a
total effective area of more than two square degrees on the sky. This is so far
the largest (spectroscopically confirmed) sample of LAEs at this redshift. We
use the secure redshifts and narrowband photometry to measure Ly$\alpha$
luminosities. We find that these LAEs span a Ly$\alpha$ luminosity range of
$\sim 2\times10^{42} - 5\times10^{43}$ erg s$^{-1}$, and include some of the
most luminous galaxies known at $z \ge 5.7$ in terms of Ly$\alpha$ luminosity.
Most of them have rest-frame equivalent widths between 20 and 300 \r{A}, and
more luminous Ly$\alpha$ emission lines tend to have broader line widths. We
detect a clear offset of $\sim20$ \r{A} between the observed Ly$\alpha$
wavelength distribution and the NB816 filter transmission curve, which can be
explained by the intergalactic medium absorption of continua blueward of
Ly$\alpha$ in the high-redshift spectra. This sample is being used to study the
Ly$\alpha$ luminosity function and galaxy properties at $z\approx5.7$.
