Candidate z~8-9 Galaxies from WFC3 Imaging
  We present a search for galaxies at 7.6<z<9.8 using the latest HST 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-alpha
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 9 more marginal but consistent with being high redshift) over an area
of ~50 square arcminutes. Previous searches for Y-drops with WFC3 have focussed
only on the Hubble Ultra Deep Field (HUDF), and our larger survey (involving
two other nearby deep fields and a wider area survey) has trebelled the number
of robust Y-drop candidates. For the first time, we have sufficient z~8-9
galaxies to fit both phi^* 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, alpha, is not well constrained. However,
adopting a similiar faint-end slope to that determined at z=3-6 (alpha=-1.7)
and a Salpeter initial mass function, then the ionizing photon budget still
falls short if f_escape<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.
