Have we detected one of the sources responsible for an early   reionisation of the Universe?
  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. 
Massimo Ricotti, Martin G. Haehnelt, Max Pettini and Martin J. Rees