Lyα Emitters at z = 5.7 in the Subaru Deep Field

We present the properties of Lyα 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 \,arcmin² 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 Lyα luminosity function derived from the photometric sample is reproduced well by a Schechter function with L^ = 7.9+3.0-2.2 × 10⁴² \,erg \,s⁻¹ and φ^ = 6.3+3.0-2.0 × 10⁻⁴ \,Mpc⁻³ for α =-1.5 (fixed) over the whole luminosity range of L 3 × 10⁴² -- 3 × 10⁴³ \,erg \,s⁻¹. We then measured the rest-frame Lyα equivalent widths for the confirmed LAEs, to find that the median among the 28 objects satisfying the photometric selection criteria is W₀ⁱ = 233 \,Å. We infer that 30%-40% of LAEs at $z=5.7$ exceed W₀ⁱ = 240 \,Å. 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 MUV -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 ~ 6 have W₀ⁱ ≥ 100 \,Å, in sharp contrast to lower-z counterparts.