Ly Alpha Emitters at z=5.7 in the Subaru Deep Field

We present the properties of Ly Alpha emitters (LAEs) at z = 5.7 in the Subaru Deep Field. A photometric sample of 89 LAE candidates is constructed from narrow-band (NB816) data down to NB816 = 26.0 (AB) in a continuous 725 arcmin^2 area. Spectra of 39 objects satisfying the photometric selection criteria for LAEs were obtained with Subaru and Keck II Telescopes, among which 28 were confirmed LAEs, one was a nearby galaxy, and eight were unclassified. We also obtained spectra of another 24 NB816-excess objects in the field, identifying six additional LAEs. We find that the Ly Alpha luminosity function derived from the photometric sample is reproduced well by a Schechter function with L* = (7.9+3.0-2.2) x 10^42 erg/s and phi* = (6.3+3.0-2.0) x 10^-4 Mpc^-3 for alpha = -1.5 (fixed) over the whole luminosity range of L ~= 3x10^42 - 3x10^43 erg/s. We then measure rest-frame Ly Alpha equivalent widths (EWs) for the confirmed LAEs, to find that the median among the 28 objects satisfying the photometric selection criteria is W_0^i = 233 A. We infer that 30% - 40% of LAEs at z=5.7 exceed W_0^i = 240 A. These large-EW objects probably cannot be accounted for by ordinary star-forming populations with a Salpeter IMF. We also find that LAEs with fainter far-UV luminosities have larger EWs. Finally, we derive the far-UV luminosity function of LAEs down to M_UV ~= -19.6 using the photometric sample, and compare it with that of Lyman-break galaxies (LBGs). We find that as high as about 80% of LBGs at z ~ 6 have W_0^i >= 100 A, in sharp contrast to lower-z counterparts.