2001ApJ...563L...5R

Lyα galaxies at high redshifts offer a powerful probe of both the formation of galaxies and the reionization of the intergalactic medium. Lyα 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. Lyα 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 τ∼5 from τ∼10⁵. This is because neutral gas scatters Lyα photons, dramatically ``blurring'' images of Lyα galaxies embedded in a neutral intergalactic medium and rendering them undetectable. We present a photometrically selected sample of redshift z~5.7 Lyα emitters derived from the Large Area Lyman Alpha survey. The presence of these low-luminosity Lyα sources immediately implies that the reionization redshift z_r>5.7. Comparing these objects with our earlier z~4.5 sample, we find that the number of z~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 Lyα line indicate young galaxies undergoing their first star formation.