The space density of redshift 5.7 Lyα emitters:: First constraints from a multislit windows search

We present results from a blind, spectroscopic search for redshift 5.7 Lyalpha 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 Angstrom atmospheric window, which contains no bright night-sky emission lines. We find nine objects with line fluxes greater than our flux limit of 6 x 10(-18) ergs s(-1) cm(-2) in our similar to5.1 arcmin(2) field. We rule out a Lyalpha identification for six of these based on the absence of the continuum break, expected at rest 1215 Angstrom 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 Lyalpha emitters brighter than 0.6(L yalpha)*, where L-Lyalphal* = 3.26 x 10(42) ergs s(-1). Selecting a field with zero Lyalpha emitters is marginally consistent with the no-evolution hypothesis - i.e., we expected to recover 2-3 Lyalpha emitters, assuming that the Lyalpha luminosity function at redshift 5.7 is the same as it is at redshift 3. Our null result rules out a brightening of L-Lyalpha* by 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 Lyalpha emitters. The paucity of z = 5.7 Lyalpha emitters raises the question of whether the Lyalpha-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 Lyalpha radiation is less than 0.4f(Lyc), where f(Lyc) is the escape fraction of Lyman continuum photons, then the star formation rate in the Lyalpha-emitting population is high enough in the no-evolution model (our upper limit) to maintain the ionization of the IGM at z = 5.7.