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Journal Articles The Astrophysical Journal Year : 2019

The Brightest z ≳ 8 Galaxies over the COSMOS UltraVISTA Field

Mauro Stefanon
Ivo Labbé
  • Function : Author
Rychard J. Bouwens
  • Function : Author
Pascal Oesch
Matthew L. N. Ashby
  • Function : Author
Karina I. Caputi
  • Function : Author
Marijn Franx
  • Function : Author
Johan P. U. Fynbo
  • Function : Author
Garth D. Illingworth
  • Function : Author
Danilo Marchesini
  • Function : Author
Bo Milvang-Jensen
  • Function : Author
Adam Muzzin
  • Function : Author
Pieter van Dokkum
  • Function : Author


We present 16 new ultrabright H AB ≲ 25 galaxy candidates at z ∼ 8 identified over the COSMOS/UltraVISTA field. The new search takes advantage of the deepest-available ground-based optical and near-infrared observations, including the DR3 release of UltraVISTA and full-depth Spitzer/IRAC observations from the SMUVS and SPLASH programs. Candidates are selected using Lyman-break color criteria, combined with strict optical non-detection and SED-fitting criteria, designed to minimize contamination by low-redshift galaxies and low-mass stars. HST/WFC3 coverage from the DASH program reveals that one source evident in our ground-based near-IR data has significant substructure and may actually correspond to 3 separate z ∼ 8 objects, resulting in a total sample of 18 galaxies, 10 of which seem to be fairly robust (with a >97% probability of being at z > 7). The UV-continuum slope β for the bright z ∼ 8 sample is β = -2.2 ± 0.6, bluer but still consistent with that of similarly bright galaxies at z ∼ 6 (β = -1.55 ± 0.17) and z ∼ 7 (β = -1.75 ± 0.18). Their typical stellar masses are {10}{9.1-0.4+0.5} M , with the SFRs of {32}-32+44{M} yr-1, specific SFR of {4}-4+8 Gyr-1, stellar ages of ∼ {22}-22+69 Myr, and low dust content {A}V={0.15}-0.15+0.30 mag. Using this sample we constrain the bright end of the z ∼ 8 UV luminosity function. When combined with recent empty field luminosity function estimates at similar redshifts, the resulting z ∼ 8 luminosity function can be equally well represented by either a Schechter or a double-power-law form. Assuming a Schechter parameterization, the best-fit characteristic magnitude is {M}* =-{20.95}-0.35+0.30 mag with a very steep faint-end slope α =-{2.15}-0.19+0.20. These new candidates include some of the brightest objects found at these redshifts, 0.5-1.0 magnitude brighter than those found over CANDELS, and providing excellent targets for spectroscopic and longer-wavelength follow-up studies.

Dates and versions

insu-03666721 , version 1 (12-05-2022)



Mauro Stefanon, Ivo Labbé, Rychard J. Bouwens, Pascal Oesch, Matthew L. N. Ashby, et al.. The Brightest z ≳ 8 Galaxies over the COSMOS UltraVISTA Field. The Astrophysical Journal, 2019, 883, ⟨10.3847/1538-4357/ab3792⟩. ⟨insu-03666721⟩
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