The discovery of two Lyman alpha 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 (A) over circle together with i' and z' broadband filters covering an 814arcmin(2) 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' - NB 921 > 1 and i' - z' > 1.3. We then obtained optical spectroscopy of nine objects in our NB 92 1 -excess sample, and identified at least two Ly a 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; rho(SFR) similar to 5.2 x 10(-4) M-circle dot 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 similar to 6.6.