The SUBARU Deep Field Project: Lymanα Emitters at a Redshift of 6.6

Abstract

We present new results of a deep optical imaging survey using a narrow band filter (NB921) centered at ${\lambda = 9196 \,\mbox{Å}}$ together with $B, V, R, i^\prime$, and $z^\prime$ broadband filters in the sky area of the Subaru Deep Field, which has been promoted as one of legacy programs of the 8.2m Subaru Telescope. We obtained a photometric sample of 58 $\mathrm{Ly}\,\alpha$ emitter candidates at $z \approx 6.5 \hbox{--} 6.6$ among $\sim 180$ strong NB921-excess ($z^\prime-\mathit{NB921} \gt 1.0 z^\prime-\mathit{NB921} > 1.0$) objects together with a color criterion of $i^\prime-z^\prime \gt 1.3 i^\prime-z^\prime > 1.3$. We then obtained optical spectra of 20 objects in our NB921-excess sample, and identified at least nine $\mathrm{Ly}\,\alpha$ emitters at $z \sim 6.5 \hbox{--} 6.6$, including the two emitters reported by Kodaira et al. (2003, PASJ, 55, L17). Since our $\mathrm{Ly}\,\alpha$-emitter candidates are free from strong amplification of gravitational lensing, we are able to discuss their observational properties from a statistical point of view. Based on these new results, we obtained a lower limit of the star-formation rate density of $\rho_\mathrm{SFR} \simeq 5.7 \times 10^{-4} \,h_{0.7} \,{{{M}_{\odot}}} \,\mathrm{yr}^{-1} \,\mathrm{Mpc}^{-3}$ at $z \approx 6.6$, being consistent with our previous estimate. We discuss the nature of star-formation activity in galaxies beyond $z=6$.