Lyα 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 $\mathit{NB816} = 26.0$ (AB) in a continuous $725 \,\mathrm{arcmin}^2$ 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 $L^\star = 7.9^{+3.0}_{-2.2} \times 10^{42} \,\mathrm{erg} \,\mathrm{s}^{-1}$ and $\phi^\star = 6.3^{+3.0}_{-2.0} \times 10^{-4} \,\mathrm{Mpc}^{-3}$ for $\alpha =-1.5$ (fixed) over the whole luminosity range of $L \simeq 3 \times 10^{42} \hbox{--} 3 \times 10^{43} \,\mathrm{erg} \,\mathrm{s}^{-1}$ . 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 $W_{0}^{\mathrm{i}} = 233 \,{Å}$ . We infer that 30%-40% of LAEs at $z=5.7$ exceed $W_{0}^{\mathrm{i}} = 240 \,{Å}$ . 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 $M_{\mathrm{UV}} \simeq-19.6$ 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 $z \sim 6$ have $W_{0}^{\mathrm{i}} \ge 100 \,{Å}$ , in sharp contrast to lower- $z$ counterparts.