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 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 6 × 10-18 ergs s-1 cm-2 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, where L ≡ 3.26 × 1042 ergs s-1. 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 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 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 0.4fLyc, where fLyc is 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.