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J. E. Rhoads (STScI), S. Malhotra (JHU)
Lyman alpha galaxies at high redshift offer a powerful probe of both reionization and of galaxy formation. Lyman alpha emission is an efficient tool for identifying young galaxies, because it is strong in chemically primitive systems with young stars. Lyman alpha galaxies also provide a robust test of the reionization epoch that is independent of Gunn-Peterson trough observations in quasar spectra: Resonant scattering of Lyman alpha photons in a neutral universe would effectively hide Lyman alpha galaxies before reionization. This test is better than the Gunn-Peterson test in that it can distinguish line center optical depths tau=5 from tau=105.
We present a photometrically selected sample of z=5.7 Lyman alpha emitters derived from the Large Area Lyman Alpha survey. The presence of these low-luminosity Lyman alpha sources at z=5.7 immediately implies that the reionization redshift was > 5.7. Comparing these objects to our earlier z=4.5 sample, we find that the number of z=5.7 sources at fixed line luminosity marginally exceeds the no-evolution expectation, but falls well short of published model predictions. Both the z=4.5 and 5.7 samples show large equivalent widths of Lyman alpha, which indicate young galaxies undergoing their first star formation.
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