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 was constructed from narrow-band 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 11 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(-2.2)(+3.0)x 10(42) ergs(-1) and phi* = 6.3(-2.0)(+3.0) x 10(-4) MpC(-3) for alpha -1.5 (fixed) over the whole luminosity range of L similar or equal to 3 x 10(42)-3 x 10(43) ergs(-1). We then measured the rest-frame Ly alpha equivalent widths for the confirmed LAEs, to find that the median among the 28 objects satisfying the photometric selection criteria is Wo(i) = 233 angstrom. We infer that 30%-40% of LAEs at z = 5.7 exceed Wo(i) = 240 .angstrom 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 derived the far-UV luminosity function of LAEs down to M-UV similar or equal to - 19.6 using the photometric sample, and compared it with that of Lyman-break galaxies (LBGs). We find that as high as about 80% of LBGs at z similar to 6 have Wo(i) >= 100 angstrom, in sharp contrast to lower-z Counterparts.