Lyman- \alpha galaxies at high redshifts offer a powerful probe of both the formation of galaxies and the reionization of the intergalactic medium .
Lyman- \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 .
Lyman- \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 \sim 5 from \tau \sim 10 ^ { 5 } .
This is because neutral gas scatters Lyman- \alpha photons , dramatically “ blurring ” images of Lyman- \alpha galaxies embedded in a neutral intergalactic medium and rendering them undetectable .

We present a photometrically selected sample of redshift z \approx 5.7 Lyman- \alpha emitters derived from the Large Area Lyman Alpha survey .
The presence of these low-luminosity Lyman- \alpha sources immediately implies that the reionization redshift z _ { r } > 5.7 .
Comparing these objects to our earlier z \approx 4.5 sample , we find that the number of z \approx 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 Lyman- \alpha line indicate young galaxies undergoing their first star formation .