The Discovery of Two Lyman α Emitters beyond Redshift 6 in the Subaru Deep Field

We performed a deep optical imaging survey using a narrow-band filter (NB921) centered at $\lambda = 9196 \,$ Å together with $i^\prime$ and $z^\prime$ broadband filters covering an $814 \,\mathrm{arcmin}^2$ area of the Subaru Deep Field. We obtained a sample of 73 strong NB921-excess objects based on the following two color criteria: $z^\prime-\mathit{NB}\,921 \gt 1$ $z^\prime-\mathit{NB}\,921 \gt 1$ and $i^\prime-z^\prime \gt 1.3$ $i^\prime-z^\prime \gt 1.3$ . We then obtained optical spectroscopy of nine objects in our NB921-excess sample, and identified at least two Ly $\alpha$ emitters at $z=6.541 \pm 0.002$ and $z=6.578 \pm 0.002$ , each of which shows the characteristic sharp cutoff together with continuum depression at wavelengths shortward of the line peak. The latter object is more distant than HCM-6A at $z=6.56$ , which is the most distant known object that has been found so far. These new data allow us to estimate the first meaningful lower limit of the star-formation rate density beyond redshift 6; $\rho_\mathrm{SFR} \sim 5.2 \times 10^{-4} \,{{{M}_{\odot}}} \,\mathrm{yr}^{-1} \,\mathrm{Mpc}^{-3}$ . Since it is expected that the actual density is several times higher than this value, our new observation reveals that a moderately high level of star formation activity already occurred at $z \sim 6.6$ .