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

Abstract

We report on the discovery of three especially bright candidate 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 ( mag) candidate 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- mag, ) 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 , consistent with the initial selection. The remaining 2/5 galaxies have a nominal . However, with HST data alone, these objects have increased probability of being at . We measure mean UV continuum slopes for the three galaxies, marginally bluer than similarly luminous in CANDELS but consistent with previous measurements of similarly luminous galaxies at . The circularized effective radius for our brightest source is 0.9 ± 0.3 kpc, similar to previous measurements for a bright galaxy and bright galaxies. Finally, enlarging our sample to include the six brightest LBGs identified over UltraVISTA (i.e., including three other sources from Labbé et al.) we estimate for the first time the volume density of galaxies at the extreme bright end ( mag) of the 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.