LARGE AREA SURVEY FOR z=7 GALAXIES IN SDF AND GOODS-N: IMPLICATIONS FOR GALAXY FORMATION AND COSMIC REIONIZATION

We present results of our large area survey for z'-band dropout galaxies at z = 7 in a 1568 arcmin(2) sky area covering the SDF and GOODS-N fields. Combining our ultra-deep Subaru/Suprime-Cam z'- and y-band (lambda(eff) = 1 mu m) images with legacy data of Subaru and Hubble Space Telescope, we have identified 22 bright z-dropout galaxies down to y = 26, one of which has a spectroscopic redshift of z = 6.96 determined from Ly alpha emission. The z = 7 luminosity function yields the best-fit Schechter parameters of phi* = 0.69(-0.55)(+2.62) x 10(-3) Mpc(-3), M-UV* = -20.10 +/- 0.76 mag, and alpha = -1.72 +/- 0.65, and indicates a decrease from z = 6 at a > 95% confidence level. This decrease is beyond the cosmic variance in our two fields, which is estimated to be a factor of less than or similar to 2. We have found that the cosmic star formation rate density drops from the peak at z = 2-3 to z = 7 roughly by a factor of similar to 10 but not larger than similar to 100. A comparison with the reionization models suggests either that the universe could not be totally ionized by only galaxies at z = 7, or more likely that properties of galaxies at z = 7 are different from those at low redshifts having, e.g., a larger escape fraction (greater than or similar to 0.2), a lower metallicity, and/or a flatter initial mass function. Our SDF z-dropout galaxies appear to form 60 Mpc long filamentary structures, and the z = 6.96 galaxy with Ly alpha emission is located at the center of an overdense region consisting of four UV bright dropout candidates, which might suggest an existence of a well-developed ionized bubble at z = 7.