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 \approx 8 - 9 ( 15 are robust and a further 9 more marginal but consistent with being high redshift ) over an area of \approx 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 \approx 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 \approx 8 - 9 , consistent with an evolution mainly in M ^ { * } .
The candidate z \approx 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 _ { \mathrm { 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 .