  It has recently been reported that the faintest galaxies at z~6-7 display
extremely blue UV continuum slopes, with a UV power-law index beta ~ -3. Such
slopes are bluer than previously reported for any other galaxy population, and
imply 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 ~2 decades in UV luminosity over the redshift
range z~4.5-8. We explore the impact of inclusion/exclusion of less robust
high-z candidates, and use the varying depths of the samples to explore the
effects of noise and selection bias. Simple data-consistency arguments suggest
that artificially blue average values of beta can result when the analysis is
extended into the deepest ~ 0.5-mag bin of these WFC3/IR-selected samples,
regardless of the actual luminosity or z range probed. By confining attention
to robust, well-detected high-z galaxy candidates, we find that the average
value of beta is consistent with -2.05 +/- 0.10 for z=5-7, and -22 < M_UV <
-18. We create and analyse a set of simulations 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 ~ -2, the typical value displayed by the bluest star-forming
galaxies at more modest z. A robust measurement of beta for the faintest
galaxies at z~7-8 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.
