The Brightest of Reionizing Galaxies Survey: Constraints on the Bright
  End of the z~8 Luminosity Function
  We report the discovery of 33 Lyman break galaxy (LBG) candidates at z~8
detected in Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) imaging as
part of the Brightest of Reionizing Galaxies (BoRG) pure-parallel survey. The
ongoing BoRG survey currently has the largest area (274 arcmin^2) with Y_098
(or Y_105), J_125, and H_160 band coverage needed to search for z~8 galaxies,
about three times the current CANDELS area, and slightly larger than what will
be the final CANDELS wide component with Y_105 data. Our sample of 33
relatively bright Y_098-dropout galaxies have J_125 band magnitudes between
25.5 and 27.4 mag. This is the largest sample of bright (J_125 <~ 27.4) z~8
galaxy candidates presented to date. Combining our dataset with the Hubble
Ultra-Deep Field (HUDF09) dataset, we constrain the rest-frame ultraviolet
galaxy luminosity function at z~8 over the widest dynamic range currently
available. The combined datasets are well fitted by a Schechter function, i.e.
\phi(L) = \phi_* (L/L_*)^{\alpha}\ e^{-(L/L_*)}, without evidence for an excess
of sources at the bright end. At 68% confidence, we derive \phi_* =
(4.3^{+3.5}_{-2.1}) \times 10^{-4} Mpc^{-3}, M_* = -20.26^{+0.29}_{-0.34}, and
a very steep faint-end slope \alpha = -1.98^{+0.23}_{-0.22}. While the best-fit
parameters still have a strong degeneracy, especially between \phi_* and M_*,
our improved coverage at the bright end has reduced the uncertainty of the
faint-end power-law slope at z~8 compared to the best previous determination at
+/-0.4. With a future expansion of the BoRG survey, combined with planned
ultradeep WFC3/IR observations, it will be possible to further reduce this
uncertainty and clearly demonstrate the steepening of the faint-end slope
compared to measurements at lower redshift, thereby confirming the key role
played by small galaxies in the reionization of the universe.
