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We report the results of a study exploring the stellar populations of 13luminous (L>L*), spectroscopically confirmed, galaxies in the redshift interval5.5<z<6.5, all with WFC3/IR and IRAC imaging from the HST/CANDELS andSpitzer/SEDS surveys. Based on fitting the observed photometry with SEDtemplates covering a wide range of different star-formation histories, and aself consistent treatment of Lyman-alpha emission, we find that the derivedstellar masses lie within the range 10^9 Msun < M*< 10^10 Msun and are robustto within a factor of two. In contrast, we confirm previous reports that theages of the stellar populations are poorly constrained. Although thebest-fitting models for three objects have ages >= 300 Myr, the degeneraciesintroduced by dust extinction mean that only two of these objects actuallyrequire a >300 Myr old stellar population to reproduce the observed photometry.Moreover, when considering only smoothly-varying star-formation histories, weobserve a clear tension between the data and models such that a galaxy SEDtemplate with an old age is often chosen in order to try and fit objects withblue UV-slopes but red UV-to-optical colours. To break this tension we exploreSED fitting with two-component models (burst plus on-going star-formation) andallow for nebular emission. On average, the inclusion of nebular emission leadsto lower stellar-mass estimates (median offset 0.18 dex), moderately higherspecific star-formation rates, and allows for a wider range of plausiblestellar ages. However, based on our SED modelling, we find no strong evidencefor extremely young ages in our sample (<50 Myr). Finally, considering all ofthe different star-formation histories explored, we find that the medianbest-fitting ages are of the order 200-300 Myr and that the objects with thetightest constraints indicate ages in the range 50-200 Myr (Abridged).

The ages, masses and star-formation rates of spectroscopically confirmed z~6 galaxies in CANDELS