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We study the physical characteristics of galaxies at 6.3 < z < 8.6, selectedfrom deep near-infrared imaging with the Wide Field Camera 3 (WFC3) on boardthe Hubble Space Telescope. Accounting for the photometric scatter usingsimulations, galaxies at z ~ 7 have bluer UV colors compared to typical localstarburst galaxies at > 4 sigma confidence. Although these colors necessitateyoung ages (<100 Myr), low or zero dust attenuation, and low metallicities,these are explicable by normal (albeit unreddened) stellar populations, with noevidence for near-zero metallicities and/or top-heavy initial mass functions.The age of the Universe at these redshifts limits the amount of stellar mass inlate-type populations, and the WFC3 photometry implies galaxy stellar masses ~10^8 - 10^9 Msol for Salpeter initial mass functions to a limiting magnitude ofM_1500 ~ -18. The masses of ``characteristic'' (L*) z > 7 galaxies are smallerthan those of L* Lyman break galaxies (LBGs) at lower redshifts, and arecomparable to less evolved galaxies selected on the basis of their Lyman alphaemission at 3 < z < 6, implying that the 6.3 < z < 8.6 galaxies are theprogenitors of more evolved galaxies at lower redshifts. We estimate that Lymanalpha emission is able to contribute to the observed WFC3 colors of galaxies atthese redshifts, with an estimated typical line flux of ~ 10^-18 erg s^-1cm^-2, roughly a factor of four below currently planned surveys. The integratedUV specific luminosity for the detected galaxies at z ~ 7 and z ~ 8 is withinfactors of a few of that required to reionize the IGM assuming low clumpingfactors, implying that in order to reionize the Universe galaxies at theseredshifts have a high ( ~ 50%) escape fraction of Lyman continuum photons,possibly substantiated by the very blue colors of this population.

On the Stellar Populations and Evolution of Star-Forming Galaxies at 6.3 < z < 8.6