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The Space Density of Redshift 5.7 Ly-Alpha Emitters

We present results from a blind, spectroscopic search for redshift 5.7 LyA emission-line galaxies at Keck I. Using a band-limiting filter and custom slitmask, the LRIS detector was covered with low resolution spectra in the 8100 - 8250 A atmospheric window which contains no bright night sky emission lines. Nine objects with line fluxes greater than our flux limit of 6e-18 erg/s/cm2 were found in the ~5.1 square arcminute field. Follow up observations indicate none of these are z=5.7 LyA emitters -- a result which places strong limits on evolution scenarios for LyA emitters between redshift 3 and redshift 5.7. In particular, the paucity of z=5.7 LyA emitters raises the question of whether the LyA -selected population plays a significant role in maintaining the ionization of the intergalactic medium at z = 5.7. We argue that if the escape fraction of LyA from galaxies is less than $0.4 f_{LyC}$, where $f_{LyC}$ is the escape fraction of Lyman continuum photons, then the star formation rate in the \lya emitting population is high enough in the no-evolution model (our upper limit) to maintain the ionization of the IGM at z=5.7.