The Luminosity Function of Lyα Emitters at Redshift z ∼ 5.7^*

We report the results of a wide-field narrowband survey for redshift z ∼ 5.7 Lyα emitters carried out with the 34' × 27' field-of-view SuprimeCam mosaic CCD camera on the Subaru 8.3 m telescope. Deep narrowband imaging of the SSA22 field through a 120 Å bandpass filter centered at a nominal wavelength of 8150 Å was combined with deep multicolor RIz' broadband imaging with SuprimeCam, then supplemented with BVRZ imaging taken with the 42' × 28' field-of-view CFH12K camera on the Canada-France-Hawaii 3.6 m telescope to select high-redshift galaxy candidates. Spectroscopic observations were made using the new wide-field multiobject DEIMOS spectrograph on the 10 m Keck II telescope for 22 of the 26 candidate objects. Eighteen of these objects were identified as z ∼ 5.7 Lyα emitters, and a further nineteenth object from the candidate list was identified based on an LRIS spectrum. At the 3.3 Å resolution of the DEIMOS observations the asymmetric profile for Lyα emission with its steep blue falloff can be clearly seen in the spectra of the identified galaxies. This is by far the largest spectroscopic sample of galaxies at these redshifts, and we use it to describe the distribution of equivalent widths and continuum color break properties for identified Lyα galaxies compared with the foreground population. The large majority (at least 75%) of the lines have rest-frame Lyα equivalent widths substantially less than 240 Å and can be understood in terms of young star-forming galaxies with a Salpeter initial mass function for the stars. With the narrowband selection criteria of I-N > 0.7 and N < 25.05 (AB magnitudes) we find a surface density of Lyα emitters of 0.03 arcmin^-2 in the filter bandpass (Δz = 0.1) down to a limiting flux of just under 2 × 10^-17 ergs cm^-2 s^-1. The luminosity function of the Lyα emitters is similar to that at lower redshifts to the lowest measurable luminosity of 10⁴³ ergs s^-1 as is the universal star formation rate based on their continuum properties. However, we note that the objects are highly structured in both their spatial and spectral properties on the angular scale of the fields (≈60 Mpc), and that multiple fields will have to be averaged to accurately measure their ensemble properties.