THE brightest of reionizing galaxies survey: Constraints on the bright end of the z ∼ 8 luminosity function

Abstract

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 (274arcmin2) 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., , 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. © 2012. The American Astronomical Society. All rights reserved.