A critical analysis of the ultraviolet continuum slopes (β) of high-redshift galaxies: No evidence (yet) for extreme stellar populations at z > 6

J. S. Dunlop, R. J. Mclure, B. E. Robertson, R. S. Ellis, D. P. Stark, M. Cirasuolo, L. De Ravel

Research output: Contribution to journalArticlepeer-review

127 Scopus citations


Following the discovery of the first significant samples of galaxies at z > 6.5 with Wide Field Camera 3/Infra-Red (WFC3/IR) on board Hubble Space Telescope (HST), it has been claimed that the faintest high-redshift galaxies display extremely blue ultraviolet (UV) continuum slopes, with a UV power-law index β≃-3 (where f λ∝λ β). Such slopes are bluer than previously reported for any other galaxy population, and are most readily explained theoretically by extinction-free, young and very low metallicity stellar populations with a high ionizing photon escape fraction. Here we undertake a critical study of the evidence for such extreme values of β, combining three new WFC3/IR-selected samples of galaxies spanning nearly two decades in UV luminosity over the redshift range z≃ 4.5-8. We explore the impact of inclusion/exclusion of less robust high-redshift candidates and use the varying depths of the samples to explore the effects of noise and selection bias at a given UV luminosity. Simple data-consistency arguments suggest that artificially blue average values of β can result when the analysis is extended into the deepest ≃0.5 mag bin of these WFC3/IR-selected galaxy samples, regardless of the actual luminosity or redshift range probed. By confining attention to robust high-redshift galaxy candidates, with at least one 8σ detection in the WFC3/IR imaging, we find that the average value of β is consistent with 〈β〉=-2.05 ± 0.10 over the redshift range z= 5-7 and the UV absolute magnitude range -22 < M UV, AB < - 18, and that 〈β〉 shows no significant trend with either redshift or M UV. We create and analyse a set of simple end-to-end simulations based on the WFC3/IR+ACS Hubble Ultra Deep Field (HUDF) and Early Release Science data sets which demonstrate that a bias towards artificially low/blue average values of β is indeed 'expected' when the UV slope analysis is extended towards the source detection threshold, and conclude that there is as yet no clear evidence for UV slopes significantly bluer than β≃-2, the typical value displayed by the bluest star-forming galaxies at more modest redshifts. A robust measurement of 〈β〉 for the faintest galaxies at z≃ 7 (and indeed z≃ 8) remains a key observational goal, as it provides a fundamental test for high escape fractions from a potentially abundant source of re-ionizing photons. This goal is achievable with HST, but requires still deeper WFC3/IR imaging in the HUDF.

Original languageEnglish (US)
Pages (from-to)901-912
Number of pages12
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
StatePublished - Feb 2012


  • Dark ages, reionization, first stars
  • Galaxies: evolution
  • Galaxies: formation
  • Galaxies: high-redshift
  • Galaxies: starburst

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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