Lyα emitters at redshift z=5.7

Ly alpha galaxies at high redshifts offer a powerful probe of both the formation of galaxies and the reionization of the intergalactic medium. Ly alpha line emission is an efficient tool for identifying young galaxies at high redshift because it is strong in systems with young stars and little or no dust-properties expected in galaxies undergoing their first burst of star formation. Ly alpha galaxies also provide a robust test of the reionization epoch that is independent of Gunn-Peterson trough observations in quasar spectra and is better able to distinguish line-center optical depths tau similar to 5 from tau similar to 10(5). This is because neutral gas scatters Ly alpha photons, dramatically blurring images of Ly alpha galaxies embedded in a neutral intergalactic medium and rendering them undetectable. We present a photometrically selected sample of redshift z approximate to 5.7 Ly alpha emitters derived from the Large Area Lyman Alpha survey. The presence of these low-luminosity Ly alpha sources immediately implies that the reionization redshift z(r) > 5.7. Comparing these objects with our earlier z approximate to 4.5 sample, we find that the number of z approximate to 5.7 emitters at fixed line luminosity marginally exceeds the no-evolution expectation but falls well short of published model predictions. The equivalent width distribution is similar at the two redshifts. The large equivalent widths of the Ly alpha line indicate young galaxies undergoing their first star formation.