HST Imaging of the Brightest z ∼ 8-9 Galaxies from UltraVISTA: The Extreme Bright End of the UV Luminosity Function

We report on the discovery of three especially bright candidate z(phot) greater than or similar to 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 less than or similar to H less than or similar to 25.5 mag) candidate z greater than or similar to 8 Lyman break galaxies (LBGs) identified over 1.6 degrees(2) of the COSMOS/UltraVISTA field. These were selected as Y and J dropouts by leveraging the deep (Y-to-K-S similar to 25.3-24.8 mag, 5 sigma) 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) similar to 8.0-8.7, consistent with the initial selection. The remaining 2/5 galaxies have a nominal z(phot) similar to 2. However, with HST data alone, these objects have increased probability of being at z similar to 9. We measure mean UV continuum slopes beta = -1.74 +/- 0.35 for the three z similar to 8-9 galaxies, marginally bluer than similarly luminous z similar to 4-6 in CANDELS but consistent with previous measurements of similarly luminous galaxies at z similar to 7. The circularized effective radius for our brightest source is 0.9 +/- 0.3. kpc, similar to previous measurements for a bright z similar to 11 galaxy and bright z similar to 7 galaxies. Finally, enlarging our sample to include the six brightest z similar to 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 similar to -22 mag) of the z similar to 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.