2012ApJ...760..108B

We report the discovery of 33 Lyman-break galaxy candidates at z ∼ 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 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 Y₁₀₅data (required to select z ∼ 8 sources). Our sample of 33 relatively bright Y ₀₉₈-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 data set with the Hubble Ultra-Deep Field data set, we constrain the rest-frame ultraviolet galaxy luminosity function at z ∼ 8 over the widest dynamic range currently available. The combined data sets 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)x10^–4 M/pc3, 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.