Detections of Faint Lyman-alpha Emitters at z = 5.7: Galaxy Building
  Blocks and Engines of Reionization
  We report results of a unprecedentedly deep, blind search for Ly-alpha
emitters (LAEs) at z = 5.7 using IMACS, the Inamori-Magellan Areal Camera &
Spectrograph, with the goal of identifying missing sources of reionization that
could also be basic building blocks for today's L* galaxies. We describe how
improvements in wide field imaging with the Baade telescope, upgrades to IMACS,
and the accumulation of ~20 hours of integration per field in excellent seeing
led to the detection of single-emission-line sources as faint as F ~ 2 x
10^{-18} ergs s^{-1} cm^{-2}, a sensitivity 5 times deeper than our first
search (Martin et al. 2008). A reasonable correction for foreground interlopers
implies a steep rise of approximately an order of magnitude in source density
for a factor of four drop in flux, from F = 10^{-17.0} ergs s^{-1} cm^{-2} to F
= 10^{-17.6} (2.5) x 10^{-18} ergs s^{-1} cm^{-2}. At this flux the putative
LAEs have reached a surface density of ~1 per sq arcminute -- a comoving volume
density of 4 x 10^{-3} Mpc^{-3}, several times the density of L* galaxies
today. Such a population of faint LAEs would account for a significant fraction
of the critical flux density required to complete reionization at this epoch,
and would be good candidates for building blocks of stellar mass ~10^{8-9} Msun
for the young galaxies of this epoch.
