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

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 -3 (where f ). 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 , 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 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 8 detection in the WFC3/IR imaging, we find that the average value of is consistent with = -2.05 0.10 over the redshift range z = 5-7 and the UV absolute magnitude range -22 < M UV,AB < -18, and that 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 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 -2, the typical value displayed by the bluest star-forming galaxies at more modest redshifts. A robust measurement of 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.