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 ~ 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 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 (required to select z ~ 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 <~ 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., \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^+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 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.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy under NASA contract NAS5-26555. These observations are associated with programs 11519, 11520, 11524, 11528, 11530, 11533, 11534, 11541, 11700, 11702, 12024, 12025, and 12572.