We present a search for galaxies at 7.6 < z < 9.8 using the latest Hubble Space Telescope/Wide Field Camera 3 (WFC3) near-infrared data, based on the Lyman-break technique. We search for galaxies which have large (Y -J) colours (the 'Y-drops') on account of the Lyman forest absorption, and with (J -H) colours inconsistent with being low-redshift contaminants. We identify 24 candidates at redshift z 8-9 (15 are robust and a further nine more marginal but consistent with being high redshift) over an area of 50 arcmin 2 . Previous searches for Y-drops with WFC3 have focused only on the Hubble Ultra Deep Field, and our larger survey (involving two other nearby deep fields and a wider area survey) has trebled the number of robust Y-drop candidates. For the first time, we have sufficient z 8-9 galaxies to fit both * and M * of the UV Schechter luminosity function. There is evidence for evolution in this luminosity function from z = 6-7 to z = 8-9, in the sense that there are fewer UV-bright galaxies at z 8-9, consistent with an evolution mainly in M * . The candidate z 8-9 galaxies we detect have insufficient ionizing flux to reionize the Universe, and it is probable that galaxies below our detection limit provide a significant UV contribution. The faint-end slope, , is not well constrained. However, adopting a similar faint-end slope to that determined at z = 3-6 ( = -1.7) and a Salpeter initial mass function (IMF), then the ionizing photon budget still falls short if f esc < 0.5, even integrating down to M UV = -8. A steeper faint-end slope or a low-metallicity population (or a top-heavy IMF) might still provide sufficient photons for star-forming galaxies to reionize the Universe, but confirmation of this might have to await the James Webb Space Telescope.
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Lorenzoni et al. (2011) studied this question.