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 arcmin 2 sky area covering the SDF and GOODS-N fields. Combining our ultra-deep Subaru/Suprime-Cam z -and y-band (λ eff = 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 −3 Mpc −3 , M * UV = −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 60 Mpc 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.

Low-mass galaxy formation and the ionizing photon budget during reionization

“…The observations in black filled circles are based on the HUDF (Bouwens et al 2007(Bouwens et al , 2011bOesch et al 2012), the squares are the HUDF and eXtreme Deep Field with GOODS-South (Oesch et al 2013), the pentagon is from HUDF and eXtreme Deep Field with both GOODS-South and North , the upward pointing triangle is from BoRG (Bradley et al 2012;Schmidt et al 2014) and the downward pointing triangle is from CLASH (Bouwens et al 2012a;Zheng et al 2012;Coe et al 2013). The bottom grey horizontal curve indicates the UV ionisation density to completely reionize the Universe (hence case 'A' recombination rates for hydrogen are used), assuming a clumping factor of 3 Shull et al 2012), escape fraction of 0.2 (Ouchi et al 2009) and a (logarithmic) ratio of Lyman continuum photons to UV flux of 25.2 ). The other grey curves are for lower escape fractions.…”

Star Forming Dwarf Galaxies

“…Today there is a strong support for the scenario that SFDGs were the main drivers of the cosmic reionization [e.g. 10,11,40,130] and there are indications [120] that the dwarf galaxies at high redshifts are both metal poor, have a flatter IMF and a higher escape rate than local galaxies. But from studies of local metal poor dwarfs that are leaking, we can better understand the physical conditions necessary for leakage.…”

Frontier Fields: Combining HST, VLT, andSpitzerdata to explore thez~ 8 Universe behind the lensing cluster MACSJ0416.1−2403

“…Ten years ago, only a dozen objects at z > 6 had been discovered (Bouwens et al 2004;Kneib et al 2004), none above z > 7.5. To date, the number of z ∼ 6, z ∼ 7, and z ≥ 8 galaxies selected in deep surveys count in the thousands (e.g., Le Fevre et al 2015;Bouwens et al 2014b;Monna et al 2014;Willott et al 2013), several hundreds (e.g., Bouwens et al 2011;Ouchi et al 2009;Ouchi et al 2013;Tilvi et al 2013) and about one hundred (e.g., Labbé et al 2013;Oesch et al 2014b;Schmidt et al 2014;Zheng et al 2012;Trenti et al 2012), respectively. Thanks to A&A 575, A92 (2015) the ever-increasing numbers of objects, the evolution and properties of galaxies is relatively well constrained up to z ∼ 6, with many secure spectroscopic confirmations (e.g., Jiang et al 2013;Willott et al 2013;Stark et al 2014;Curtis-Lake et al 2013;Schenker et al 2012;Le Fevre et al 2015).…”