  We present the results of a systematic search for galaxies in the redshift
range z = 6 - 9, within the new, deep, near-infrared imaging of the Hubble
Ultra Deep Field provided by the Wide Field Camera 3 (WFC3) on HST. We have
performed full SED fitting to the optical+infrared photometry of all
high-redshift galaxy candidates detected at greater than 5-sigma in at least
one of the WFC3/IR broad-band filters. After rejection of contaminants, the
result is a sample of 49 galaxies with primary redshift solutions z > 5.9. Our
sample, selected without recourse to specific colour cuts, re-selects all but
the faintest one of the 16 z-drops selected by Oesch et al. (2009), recovers
all 5 of the Y-drops reported by Bouwens et al. (2009), and adds a further 29
galaxy candidates, of which 12 lie beyond z = 6.3, and 4 lie beyond z = 7. We
also present confidence intervals on our photometric redshift estimates, and
caution that acceptable low-redshift (z < 2) solutions exist for 28 out of the
37 galaxies at z > 6.3, and for all 8 galaxy candidates at z > 7.5.
Nevertheless, the very highest redshift candidates appear to be strongly
clustered in the field. We derive new estimates of the ultraviolet galaxy
luminosity function at z = 7 and z = 8. Where our results are most robust, at a
characteristic luminosity M(1500) ~ -19.5 (AB), we find that the comoving
number density of galaxies declines by a factor of ~ 2.5 between z = 6 and z =
7, and by a further factor of ~ 2 by z = 8. These results suggest that it is
difficult for the observed population of high-redshift star-forming galaxies to
achieve reionisation by z ~ 6 without a significant contribution from galaxies
well below the detection limits, plus alterations in the escape fraction of
ionising photons and/or continued vigorous star formation at z > 15.
