Accepted to The Astrophysical Journal Preprint typeset using LATEX style emulateapj v. 5/2/11 THE BRIGHTEST OF REIONIZING GALAXIES SURVEY: CONSTRAINTS ON THE BRIGHT END OF THE

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 arcmin2) with Y098 (or Y105), J125, and H160 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 Y105 data (required to select z ∼ 8 sources). Our sample of 33 relatively bright Y098-dropout galaxies have J125 band magnitudes between 25.5 and 27.4 mag. This is the largest sample of bright (J125. 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. φ(L) = φ∗(L/L∗)α e−(L/L∗), without evidence for an excess of sources at the bright end. At 68 % confidence, for h = 0.7 we derive φ ∗ = (4.3+3.5−2.1)×10−4 Mpc−3, M ∗ = −20.26+0.29−0.34, and a very steep faint-end slope α = −1.98+0.23−0.22. While the best-fit parameters still have a strong degeneracy, especially between φ ∗ 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