2017ApJ...851...43S

We report on the discovery of three especially bright candidate z_phot 8 galaxies. Five sources were targeted for follow-up with the Hubble Space Telescope (HST)/Wide Field Camera 3 (WFC3), selected from a larger sample of 16 bright (24.8 H 25.5 mag) candidate z 8 Lyman break galaxies (LBGs) identified over 1.6 degrees² of the COSMOS/UltraVISTA field. These were selected as Y and J dropouts by leveraging the deep (Y-to-K_S∼25.3–24.8 mag, 5σ ) NIR data from the UltraVISTA DR3 release, deep ground-based optical imaging from the CFHTLS and Suprime-Cam programs, and Spitzer/IRAC mosaics combining observations from the SMUVS and SPLASH programs. Through the refined spectral energy distributions, which now also include new HyperSuprimeCam g-, r-, i-, z-, and Y-band data, we confirm that 3/5 galaxies have robust z_phot∼8.0–8.7, consistent with the initial selection. The remaining 2/5 galaxies have a nominal z_phot∼2. However, with HST data alone, these objects have increased probability of being at z∼9. We measure mean UV continuum slopes β=-1.74±0.35 for the three z∼8–9 galaxies, marginally bluer than similarly luminous z∼4–6 in CANDELS but consistent with previous measurements of similarly luminous galaxies at z∼7. The circularized effective radius for our brightest source is 0.9 ± 0.3 kpc, similar to previous measurements for a bright z∼11 galaxy and bright z∼7 galaxies. Finally, enlarging our sample to include the six brightest z∼8 LBGs identified over UltraVISTA (i.e., including three other sources from Labbe et al.) we estimate for the first time the volume density of galaxies at the extreme bright end (M_UV∼-22 mag) of the z∼8 UV luminosity function. Despite this exceptional result, the still large statistical uncertainties do not allow us to discriminate between a Schechter and a double-power-law form.