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 λ = 9196 \,Å together with i^ and z^ broadband filters covering an 814 \,arcmin² area of the Subaru Deep Field. We obtained a sample of 73 strong NB921-excess objects based on the following two color criteria:z⁻NB\,921 > 1z^-NB\,921 > 1$ and $i^-z^ > 1.3i⁻z^ > 1.3. We then obtained optical spectroscopy of nine objects in our NB921-excess sample, and identified at least two Lyα emitters at z=6.541 ± 0.002 and z=6.578 ± 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; ρSFR ~ 5.2 × 10⁻⁴ \,M_ \,yr⁻¹ \,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 ~ 6.6.