The Discovery of Two Lyman$\alpha$ Emitters Beyond Redshift 6 in the Subaru Deep Field

We have performed a deep optical imaging survey using a narrowband filter ($NB921$) centered at $\lambda =$ 9196 \AA ~ together with $i^{\mathrm{\prime }}$ and $z^{\mathrm{\prime }}$ broadband filters covering an 814 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; $z^{\mathrm{\prime }}-NB921>1$ and $i^\prime - z^\prime > 1.3$\mathrm{.}Wethenobtainedopticalspectroscopyofnineobjectsinour$NB921$-excesssample,andidentifiedatleasttwoLy$\alpha$ 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 the continuum depression at wavelengths shortward of the line peak. The latter object is more distant than HCM-6A at $z=6.56$ and thus this is the most distant known object found so far. These new data allow us to estimate the first meaningful lower limit of the star formation rate density beyond redshift 6; $\rho_{\rm SFR} \sim 5.2 \times 10^{-4} M_\odot$yr$^{-1}$Mpc$^{-3}$. Since it is expected that the actual density is higher by a factor of several than this value, our new observation reveals that a moderately high level of star formation activity already occurred at $z\sim$ 6.6.