Detections of faint Ly[alpha] emitters at z = 5.7: Galaxy building blocks and engines of reionization

Abstract

We report results of an unprecedentedly deep, blind search for Ly[alpha] 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[superscript ∗] 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 [approximate to] 2 × 10[superscript −18] erg s[superscript −1] cm[superscript −2], 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[superscript −17.0] erg s[superscript −1] cm[superscript −2] to F = 10[superscript −17.6] (2.5 × 10[superscript −18]) erg s[superscript −1] cm[superscript −2]. At this flux the putative LAEs have reached a surface density of ∼1 arcmin[superscript −2]—a comoving volume density of 4 × 10[superscript −3] Mpc[superscript −3], several times the density of L[superscript *] 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[superscript 8]–10[superscript 9] M[subscript circled dot] for the young galaxies of this epoch.