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 candidates at z similar to 8 detected in Hubble Space Telescope 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 similar to 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 (required to select z similar to 8 sources). 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) less than or similar to 27.4) z similar to 8 galaxy candidates presented to date. Combining our data set with the Hubble Ultra-Deep Field data set, we constrain the rest-frame ultraviolet galaxy luminosity function at z similar to 8 over the widest dynamic range currently available. The combined data sets 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, for h = 0.7 we derive phi(*) = (4.3(-2.1)(+3.5)) x 10(-4) Mpc(-3), M-* = -20.26(-0.34)(+0.29), and a very steep faint-end slope alpha = -1.98(-0.22)(+0.23). 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 similar to 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.