The Subaru Deep Field project:: Lyman α emitters at a redshift of 6.6

We present new results of a deep optical imaging survey using a narrow band filter (NB921) centered at lambda = 9196 angstrom together with B, V, R, i', and z' broadband filters in the sky area of the Subaru Deep Field, which has been promoted as one of legacy programs of the 8.2 m Subaru Telescope. We obtained a photometric sample of 58 Ly alpha emitter candidates at z approximate to 6.5-6.6 among similar to 180 strong NB921-excess (z' - NB921 > 1.0) objects together with a color criterion of i' - z' > 1.3. We then obtained optical spectra of 20 objects in our NB921-excess sample, and identified at least nine Ly alpha emitters at z similar to 6.5-6.6, including the two emitters reported by Kodaira et al. (2003, PASJ, 55, L17). Since our Ly alpha-emitter candidates are free from strong amplification of gravitational lensing, we are able to discuss their observational properties from a statistical point of view. Based on these new results, we obtained a lower limit of the star-formation rate density of rho(SFR) similar or equal to 5.7 x 10(-4) h(0.7) M-circle dot yr(-1) Mpc(-3) at z approximate to 6.6, being consistent with our previous estimate. We discuss the nature of star-formation activity in galaxies beyond z = 6.