ON THE STELLAR POPULATIONS AND EVOLUTION OF STAR-FORMING GALAXIES AT 6.3 < z ≤ 8.6

Observations of very distant galaxies probe both the formation and evolution of galaxies, and also the nature of the sources responsible for reionizing the intergalactic medium (IGM). Here, we study the physical characteristics of galaxies at 6.3 < z <= 8.6, selected from deep near-infrared imaging with the Wide Field Camera 3 (WFC3) on board the Hubble Space Telescope. We investigate the rest-frame ultraviolet (UV) colors, stellar masses, ages, metallicities, and dust extinction of this galaxy sample. Accounting for the photometric scatter using simulations, galaxies at z similar to 7 have bluer UV colors compared to typical local starburst galaxies at >4 sigma confidence. Although the blue colors of galaxies at these redshifts necessitate young ages (<100 Myr), low or zero dust attenuation, and low metallicities, these are explicable by normal (albeit unreddened) stellar populations, with no evidence for near-zero metallicities and/or top-heavy initialmass functions. Most of these galaxies are undetected in deep Spitzer Infrared Array Camera imaging. However, the age of the universe at these redshifts limits the amount of stellar mass in late-type populations, and the WFC3 photometry implies galaxy stellar masses similar to 10(8)-10(9) M(circle dot) for Salpeter initial mass functions to a limiting magnitude of M(1500) similar to -18. The masses of "characteristic" (L*) z > 7 galaxies are smaller than those of L* Lyman break galaxies at lower redshifts, and are comparable to less evolved galaxies selected on the basis of their Ly alpha emission at 3 < z < 6, implying that the 6.3 < z <= 8.6 galaxies are the progenitors of more evolved galaxies at lower redshifts. We estimate that Ly alpha emission is able to contribute to the observed WFC3 colors of galaxies at these redshifts, with an estimated typical line flux of approximate to 10(-18) erg s(-1) cm(-2), roughly a factor of 4 below currently planned surveys. The integrated UV specific luminosity for the detected galaxies at z similar to 7 and z similar to 8 is within factors of a few of that required to reionize the IGM assuming low clumping factors, even with no correction for luminosity incompleteness. This implies that in order to reionize the universe, galaxies at these redshifts have a high (similar to 50%) escape fraction of Lyman continuum photons, possibly substantiated by the very blue colors of this population.