Steven L.  Finkelstein (TAMU), Casey Papovich (TAMU), Henry
C. Ferguson (STScI), Mark Dickinson (NOAO), Mauro Giavalisco (UMass),
Naveen A. Reddy (NOAO) & Anton M. Koekemoer (STScI)

Observations of 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.  Galaxies at z ~ 7 have bluer UV
colors compared to typical local starburst galaxies at > 4 sigma
confidence.  However, these colors are explicable by normal (albeit
unreddened) stellar populations similar to the local galaxy NGC 1705,
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.  These
masses are smaller than those of typical Lyman break galaxies 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 these
6.3 < z < 8.6 galaxies are the building blocks of more evolved
galaxies at lower redshifts.  Lastly, we find that the integrated UV
specific luminosity for our sample implies 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.