  We present the results of a photometric redshift analysis designed to
identify z>6 galaxies from the near-IR HST imaging in three deep fields (HUDF,
HUDF09-2 & ERS). By adopting a rigorous set of criteria for rejecting low-z
interlopers, and by employing a deconfusion technique to allow the available
IRAC imaging to be included in the candidate selection process, we have derived
a robust sample of 70 Lyman-break galaxies (LBGs) spanning the redshift range
6.0<z<8.7. Based on our final sample we investigate the distribution of UV
spectral slopes (beta), finding a variance-weighted mean value of <beta>=-2.05
+/- 0.09 which, contrary to some previous results, is not significantly bluer
than displayed by lower-redshift starburst galaxies. We confirm the correlation
between UV luminosity and stellar mass reported elsewhere, but based on fitting
galaxy templates featuring a range of star-formation histories, metallicities
and reddening we find that, at z>=6, the range in mass-to-light ratio (M*/L_UV)
at a given UV luminosity could span a factor of ~50. Focusing on a sub-sample
of twenty-one candidates with IRAC detections at 3.6-microns we find that L*
LBGs at z~6.5 have a median stellar mass of M* = (2.1 +/- 1.1) x 10^9 Msun and
a median specific star-formation rate of 1.9 +/- 0.8 Gyr^-1. Using the same
sub-sample we have investigated the influence of nebular continuum and line
emission, finding that for the majority of candidates (16 out of 21) the
best-fitting stellar-mass estimates are reduced by less than a factor of 2.5.
Finally, a detailed comparison of our final sample with the results of previous
studies suggests that, at faint magnitudes, several high-redshift galaxy
samples in the literature are significantly contaminated by low-redshift
interlopers (abridged).
