2004ApJ...603..414M

We present results from a blind, spectroscopic search for redshift 5.7 Lyα emission-line galaxies at Keck I. Using a band-limiting filter and custom slit mask, we cover the LRIS detector with low-resolution spectra in the 8100-8250 Å atmospheric window, which contains no bright night-sky emission lines. We find nine objects with line fluxes greater than our flux limit of 6x10^–18 ergs/s/cm2 in our ∼5.1 arcmin² field. We rule out a Lyα identification for six of these based on the absence of the continuum break, expected at rest 1215 Å for high-z galaxies and/or the identification of additional emission lines in our follow-up spectra. We find that extremely metal-poor, foreground emission-line galaxies are the most difficult type of interloper to recognize. For the three remaining emission-line objects, we identify a plausible counterpart for each object in a deep V-band image of the field, suggesting that none of them has a continuum break in the i band. Our preliminary conclusion is that our field contains no z=5.7 Lyα emitters brighter than 0.6L^*_Lyα, where L^*_Lyα≡3.26x10⁴² ergs/s. Selecting a field with zero Lyα emitters is marginally consistent with the no-evolution hypothesis–i.e., we expected to recover 2-3 Lyα emitters, assuming that the Lyα luminosity function at redshift 5.7 is the same as it is at redshift 3. Our null result rules out a brightening of L^*_Lyαby more than a factor of 1.7 from redshift 3 to 5.7, or, over the same redshift interval, an increase of more than a factor of 2.2 in the number density of Lyα emitters. The paucity of z=5.7 Lyα emitters raises the question of whether the Lyα-selected population plays a significant role in maintaining the ionization of the intergalactic medium (IGM) at z=5.7. We find that if the escape fraction of Lyα radiation is less than 0.4f_Lyc, where f_Lycis the escape fraction of Lyman continuum photons, then the star formation rate in the Lyα-emitting population is high enough in the no-evolution model (our upper limit) to maintain the ionization of the IGM at z=5.7.