Following the discovery of the first significant samples of galaxies at z > 6.5 with Wide Field Camera 3/Infrared ( 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 \beta \simeq - 3 ( where f _ { \lambda } ~ { } \propto~ { } \lambda ^ { \beta } ) .
Such slopes are bluer than previously reported for any other galaxy population , and are most readily explained theoretically by extinction-free , young , 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 \beta , combining three new WFC3/IR-selected samples of galaxies spanning nearly two decades in UV luminosity over the redshift range z \simeq 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 ultraviolet luminosity .
Simple data-consistency arguments suggest that artificially blue average values of \beta can result when the analysis is extended into the deepest \simeq 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- \sigma detection in the WFC3/IR imaging , we find that the average value of \beta is consistent with \langle \beta \rangle = -2.05 \pm 0.10 over the redshift range z = 5 - 7 , and the UV absolute magnitude range -22 < M _ { UV,AB } < -18 , and that \langle \beta \rangle 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 datasets which demonstrate that a bias towards artifically low/blue average values of \beta 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 \beta \simeq - 2 , the typical value displayed by the bluest star-forming galaxies at more modest redshifts .
A robust measurement of \langle \beta \rangle for the faintest galaxies at z \simeq 7 ( and indeed z \simeq 8 ) remains a key observational goal , as it provides a fundamental test for high escape fractions from a potentially abundant source of reionizing photons .
This goal is achievable with HST , but requires still deeper WFC3/IR imaging in the HUDF .