Unveiling the nature of bright z ~ 7 galaxies with the Hubble Space Telescope

R. A. A. Bowler, J. S. Dunlop, R. J. McLure, D. J. McLeod

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We present new Hubble Space Telescope/Wide Field Camera 3 imaging of 25 extremely luminous (-23.2 <M_ UV <-21.2) Lyman-break galaxies (LBGs) at z ~ 7. The sample was initially selected from 1.65 deg^2 of ground-based imaging in the UltraVISTA/COSMOS and UDS/SXDS fields, and includes the extreme Lyman-alpha emitters, `Himiko' and `CR7'. A deconfusion analysis of the deep Spitzer photometry available suggests that these galaxies exhibit strong rest-frame optical nebular emission lines (EW_0(H_beta + [OIII]) > 600A). We find that irregular, multiple-component morphologies suggestive of clumpy or merging systems are common (f_multi > 0.4) in bright z ~ 7 galaxies, and ubiquitous at the very bright end (M_UV <-22.5). The galaxies have half-light radii in the range r_1/2 ~ 0.5-3 kpc. The size measurements provide the first robust determination of the size-luminosity relation at z ~ 7 extending to M_UV ~ -23, which we find to be steep with r_1/2 ~ L^1/2. Excluding clumpy, multi-component galaxies however, we find a shallower relation that implies an increased star-formation rate surface density in bright LBGs. Using the new, independent, HST/WFC3 data we confirm that the rest-frame UV luminosity function at z ~ 7 favours a power-law decline at the bright-end, compared to an exponential Schechter function drop-off. Finally, these results have important implications for the Euclid mission, which we predict will detect > 1000 similarly bright galaxies at z ~ 7. Our new HST imaging suggests that the vast majority of these galaxies will be spatially resolved by Euclid, mitigating concerns over dwarf star contamination.
Original languageEnglish
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
Early online date20 Dec 2016
Publication statusPublished - Apr 2017


  • Astrophysics - Astrophysics of Galaxies
  • Astrophysics - Cosmology and Nongalactic Astrophysics


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