2011ApJ...740...71D

We report results of an unprecedentedly deep, blind search for Lyα emitters (LAEs) at z = 5.7 using the Inamori-Magellan Areal Camera & Spectrograph (IMACS), 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 hr of integration per field in excellent seeing led to the detection of single-emission-line sources as faint as F ~ 2x10^–18 erg/s/cm2, a sensitivity five times deeper than our first search. 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 erg/s/cm2 to F = 10-17.6^ (2.5x10^–18) erg/s/cm2. At this flux the putative LAEs have reached a surface density of ∼1/arcmin2–a comoving volume density of 4x10^–3/Mpc3, 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 ∼10⁸-10⁹ M_☉ for the young galaxies of this epoch.