The Bright End of the UV Luminosity Function at z~8: New Constraints
  from CANDELS Data in GOODS-South
  We present new z~8 galaxy candidates from a search over ~95 arcmin^2 of
WFC3/IR data, tripling the previous search area for bright z~8 galaxies. Our
analysis uses newly acquired WFC3/IR imaging data from the CANDELS Multi-Cycle
Treasury program over the GOODS South field. These new data are combined with
existing deep optical ACS imaging to search for relatively bright (M_UV < -19.5
mag) z~8 galaxy candidates using the Lyman Break technique. These new
candidates are used to determine the bright end of the UV luminosity function
(LF) of star-forming galaxies at z~8. To minimize contamination from lower
redshift galaxies, we make full use of all optical ACS data and impose strict
non-detection criteria based on an optical chi^2_opt flux measurement. In the
whole search area we identify 16 candidate z~8 galaxies, spanning a magnitude
range H_160 = 25.7-27.9 mag. The new data show that the UV LF is a factor ~1.7x
lower at M_UV < -19.5 mag than determined from the HUDF09 and ERS data alone.
Combining this new sample with the previous candidates from the HUDF09 and ERS
data allows us to perform the most accurate measurement of the z~8 UV LF yet.
Schechter function fits to the combined data result in a best-fit
characteristic magnitude of M*(z=8) = -20.04+-0.46 mag. The faint-end slope is
very steep, though quite uncertain, with alpha = -2.06+-0.32. A combination of
wide area data with additional ultra-deep imaging will be required to
significantly reduce the uncertainties on these parameters in the future.
