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

Stefanon, M., Labbé, I., Bouwens, R. J., Brammer, G. B., Oesch, P., Franx, M., Fynbo, J. P. U., Milvang-Jensen, B., Muzzin, A., Illingworth, G. D., Le Fèvre, O., Caputi, K. I., Holwerda, B. W., McCracken, H. J., Smit, R. & Magee, D., 10-Dec-2017, In : The Astrophysical Journal. 851, 1, 19 p., 43.

Research output: Contribution to journalArticleAcademicpeer-review

  • Mauro Stefanon
  • Ivo Labbé
  • Rychard J. Bouwens
  • Gabriel B. Brammer
  • Pascal Oesch
  • Marijn Franx
  • Johan P. U. Fynbo
  • Bo Milvang-Jensen
  • Adam Muzzin
  • Garth D. Illingworth
  • Olivier Le Fèvre
  • Karina I. Caputi
  • Benne W. Holwerda
  • Henry J. McCracken
  • Renske Smit
  • Dan Magee
We report on the discovery of three especially bright candidate {z}{phot}≳ 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≲ H≲ 25.5 mag) candidate z≳ 8 Lyman break galaxies (LBGs) identified over 1.6 degrees2 of the COSMOS/UltraVISTA field. These were selected as Y and J dropouts by leveraging the deep (Y-to-{K}{{S}}∼ 25.3{--}24.8 mag, 5σ ) 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}∼ 8.0{--}8.7, consistent with the initial selection. The remaining 2/5 galaxies have a nominal {z}{phot}∼ 2. However, with HST data alone, these objects have increased probability of being at z∼ 9. We measure mean UV continuum slopes β =-1.74+/- 0.35 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 ± 0.3 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 Labbé et al.) we estimate for the first time the volume density of galaxies at the extreme bright end ({M}{UV}∼ -22 mag) of the z∼ 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.
Original languageEnglish
Article number43
Number of pages19
JournalThe Astrophysical Journal
Issue number1
Publication statusPublished - 10-Dec-2017


  • galaxies: evolution, galaxies: formation, galaxies: high-redshift, galaxies: luminosity function, mass function, STAR-FORMING GALAXIES, HUBBLE DEEP FIELD, EVOLUTION SURVEY COSMOS, HIGH-REDSHIFT GALAXIES, FRONTIER FIELDS, FORMATION RATES, STELLAR MASSES, REIONIZATION, PHOTOMETRY, DENSITY

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