The Brightest z ≳ 8 Galaxies over the COSMOS UltraVISTA Field

Stefanon, M., Labbé, I., Bouwens, R. J., Oesch, P., Ashby, M. L. N., Caputi, K. I., Franx, M., Fynbo, J. P. U., Illingworth, G. D., Le Fèvre, O., Marchesini, D., McCracken, H. J., Milvang-Jensen, B., Muzzin, A. & van Dokkum, P., 20-Sep-2019, In : The Astrophysical Journal. 883, 1, 31 p., 99.

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  • The Brightestz8 Galaxies over the COSMOS UltraVISTA Field

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  • Mauro Stefanon
  • Ivo Labbé
  • Rychard J. Bouwens
  • Pascal Oesch
  • Matthew L. N. Ashby
  • Karina I. Caputi
  • Marijn Franx
  • Johan P. U. Fynbo
  • Garth D. Illingworth
  • Olivier Le Fèvre
  • Danilo Marchesini
  • Henry J. McCracken
  • Bo Milvang-Jensen
  • Adam Muzzin
  • Pieter van Dokkum
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.
Original languageEnglish
Article number99
Number of pages31
JournalThe Astrophysical Journal
Issue number1
Publication statusPublished - 20-Sep-2019


  • galaxies: evolution, galaxies: formation, galaxies: high-redshift, galaxies: luminosity function, mass function

ID: 102491481