  In a recent paper Pell'o et al. have reported a candidate z=10 galaxy,
A1835#1916, which was found in a near-infrared survey of the central regions of
the gravitational lensing cluster A1835. If this detection is confirmed and the
detection rate turns out to be typical, then the volume averaged ultraviolet
emissivity must be rising rapidly with increasing redshift. For a magnification
due to gravitational lensing by a factor M>25 estimated by Pell'o et al., the
inferred star formation rate density at z=10 would be about one order of
magnitude higher than estimates of the star formation rate density at z=6.
Objects at z=10 would contribute substantially to the total source counts at
1.6 micronm and the estimated space density of sources may exceed the space
density of dark matter haloes in a LCDM model. We therefore argue that if
A1835#1916 is indeed at z=10 then either the magnification factor may have been
overestimated or the galaxy has a top-heavy initial mass function. Sources with
the ultraviolet flux and space density of A1835#1916 may be sufficient to
reionise most of the diffuse hydrogen in the Universe at z=10. We further use a
correlation between the equivalent width and the redshift of the Ly-alpha
emission line with respect to the systemic redshift observed in Lyman break
galaxies to obtain constraints on the ionisation state of the surrounding
intergalactic medium (IGM) from the Gunn-Peterson absorption. These constraints
also argue in favour of the surrounding IGM being partially or fully ionised.
