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

Change log
Stefanon, M 
Labbé, I 
Bouwens, RJ 
Brammer, GB 
Oesch, P 

We report on the discovery of three especially bright candidate zphot≳8 galaxies. Five sources were targeted for follow-up with HST/WFC3, selected from a larger sample of 16 bright (24.8≲H≲25.5~mag) candidate z≳8 LBGs identified over the 1.6 degrees2 of the COSMOS/UltraVISTA field. These were identified as Y and J dropouts by leveraging the deep (Y-to-KS∼25.3−24.8~mag, 5σ) NIR data from the UltraVISTA DR3 release, deep ground based optical imaging from the CFHTLS and Subaru 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 HyperSuprime Cam g, r, i, z and Y band data, we confirm that 3/5 galaxies have robust zphot∼8.0−8.7, consistent with the initial selection. The remaining 2/5 galaxies have a nominal zphot∼2. However, if we use the HST data alone, these objects have increased probability of being at z∼9. Furthermore, we measure mean UV continuum slopes β=−1.91±0.26 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±0.2 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. 2017, in prep.) we estimate for the first time the volume density of galaxies at the extreme bright (MUV∼−22~mag) end of the z∼8 UV LF. Despite this exceptional result, the still large statistical uncertainties do not allow us to discriminate between a Schechter and a double power-law form.

galaxies: evolution, galaxies: formation, galaxies: high, redshift, galaxies: luminosity function, mass function
Journal Title
Astrophysical Journal
Conference Name
Journal ISSN
Volume Title
American Astronomical Society
All rights reserved