The ages, masses and star-formation rates of spectroscopically confirmed
  z~6 galaxies in CANDELS
  We report the results of a study exploring the stellar populations of 13
luminous (L>L*), spectroscopically confirmed, galaxies in the redshift interval
5.5<z<6.5, all with WFC3/IR and IRAC imaging from the HST/CANDELS and
Spitzer/SEDS surveys. Based on fitting the observed photometry with SED
templates covering a wide range of different star-formation histories, and a
self consistent treatment of Lyman-alpha emission, we find that the derived
stellar masses lie within the range 10^9 Msun < M*< 10^10 Msun and are robust
to within a factor of two. In contrast, we confirm previous reports that the
ages of the stellar populations are poorly constrained. Although the
best-fitting models for three objects have ages >= 300 Myr, the degeneracies
introduced by dust extinction mean that only two of these objects actually
require a >300 Myr old stellar population to reproduce the observed photometry.
Moreover, when considering only smoothly-varying star-formation histories, we
observe a clear tension between the data and models such that a galaxy SED
template with an old age is often chosen in order to try and fit objects with
blue UV-slopes but red UV-to-optical colours. To break this tension we explore
SED fitting with two-component models (burst plus on-going star-formation) and
allow for nebular emission. On average, the inclusion of nebular emission leads
to lower stellar-mass estimates (median offset 0.18 dex), moderately higher
specific star-formation rates, and allows for a wider range of plausible
stellar ages. However, based on our SED modelling, we find no strong evidence
for extremely young ages in our sample (<50 Myr). Finally, considering all of
the different star-formation histories explored, we find that the median
best-fitting ages are of the order 200-300 Myr and that the objects with the
tightest constraints indicate ages in the range 50-200 Myr (Abridged).
