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=1um) images with
legacy data of Subaru and HST, 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 Lya emission. The z=7 luminosity function (LF) yields the best-fit
Schechter parameters of phi*=0.69 +2.62/-0.55 x10^(-3) Mpc^(-3), Muv*=-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 <~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 ~10 but not larger than ~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
(>~0.2), and/or a flatter IMF. Our SDF z-dropout galaxies appear to form 60-Mpc
long filamentary structures, and the z=6.96 galaxy with Lya 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.
