Probing Stellar Populations at z ~ 7 - 8
  In this proceeding we present the results from a study of very high-redshift
galaxies with the newly commissioned Wide Field Camera 3 on the Hubble Space
Telescope. With the deepest near-infrared data ever taken, we discovered 31
galaxies at 6.3 < z < 8.6. The rest-frame ultraviolet (UV) colors of these
galaxies are extremely blue, showing significant (> 4 sigma) evolution from z ~
3, over only 1 Gyr of cosmic time. While we cannot yet diagnose the exact cause
of the bluer colors, it appears a low dust content is the primary factor. The
stellar masses of these galaxies are less than comparably selected galaxies at
3 < z < 6, highlighting evolution in the stellar mass of characteristic (L*)
galaxies with redshift. Lastly, the measured rest-UV luminosity density of
galaxies in our sample seems sufficient to sustain reionization at z ~ 7 when
we account for the likely contribution from galaxies below our magnitude limit.
