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 arcmin²area of the Subaru Deep Field. We obtained a sample of 73 strongNB921-excess objects based on the following two color criteria: z - NB921 > 1 andi' - z'> 1.3.We then obtained optical spectroscopy of nine objects in our NB921-excess sample, and identified at least two Lyα emitters atz=6.541 ± 0.002 andz=6.578 ± 0.002, each of which shows the characteristic sharp cutofftogether 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^-4M_⊙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.