On the Stellar Populations and Evolution of Star-Forming Galaxies at 6.3
  < z < 8.6
  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. Accounting for the photometric scatter using
simulations, galaxies at z ~ 7 have bluer UV colors compared to typical local
starburst galaxies at > 4 sigma confidence. Although these colors 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 initial mass functions.
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 ~
10^8 - 10^9 Msol for Salpeter initial mass functions to a limiting magnitude of
M_1500 ~ -18. The masses of ``characteristic'' (L*) z > 7 galaxies are smaller
than those of L* Lyman break galaxies (LBGs) at lower redshifts, and are
comparable to less evolved galaxies selected on the basis of their Lyman 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 Lyman
alpha emission is able to contribute to the observed WFC3 colors of galaxies at
these redshifts, with an estimated typical line flux of ~ 10^-18 erg s^-1
cm^-2, roughly a factor of four below currently planned surveys. The integrated
UV specific luminosity for the detected galaxies at z ~ 7 and z ~ 8 is within
factors of a few of that required to reionize the IGM assuming low clumping
factors, implying that in order to reionize the Universe galaxies at these
redshifts have a high ( ~ 50%) escape fraction of Lyman continuum photons,
possibly substantiated by the very blue colors of this population.
