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 (NB 921) 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 NB 921-excess objects based on the following two color criteria: z′ - NB921 > 1 and i′ - z′ > 1.3. We then obtained optical spectroscopy of nine objects in our NB 921 -excess sample, and identified at least two Ly α 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; ρ_SFR ∼ 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.