Large area survey for Z = 7 galaxies in SDF and goods-N: Implications for galaxy formation and cosmic reionization

Abstract

We present results of our large area survey for z′-band dropout galaxies at z = 7 in a 1568 arcmin2 sky area covering the SDF and GOODS-N fields. Combining our ultra-deep Subaru/Suprime-Cam z′- and y-band (λeff = 1 μm) 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 Lyα emission. The z = 7 luminosity function yields the best-fit Schechter parameters of φ= 0.69+2.62-0.55 × 10 -3Mpc-3, MUV = -20.10 0.76 mag, and α = -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), a lower metallicity, and/or a flatter initial mass function. Our SDF z-dropout galaxies appear to form 60Mpc long filamentary structures, and the z = 6.96 galaxy with Lyα 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. © 2009. The American Astronomical Society. All rights reserved.