We present a new determination of the UV galaxy luminosity function ( LF ) at redshift z \simeq 7 and z \simeq 8 , and a first estimate at z \simeq 9 .
An accurate determination of the form and evolution of the galaxy LF during this era is of key importance for improving our knowledge of the earliest phases of galaxy evolution and the process of cosmic reionization .
Our analysis exploits to the full the new , deepest WFC3/IR imaging from our Hubble Space Telescope ( HST ) Ultra Deep Field 2012 ( UDF12 ) campaign , with dynamic range provided by including a new and consistent analysis of all appropriate , shallower/wider-area HST survey data .
Our new measurement of the evolving LF at z \simeq 7 - 8 is based on a final catalogue of \simeq 600 galaxies , and involves a step-wise maximum likelihood determination based on the photometric-redshift probability distribution for each object ; this approach makes full use of the 11 -band imaging now available in the Hubble Ultra Deep Field ( HUDF ) , including the new UDF12 F140W data , and the latest Spitzer IRAC imaging .
The final result is a determination of the z \simeq 7 LF extending down to UV absolute magnitudes M _ { 1500 } = -16.75 ( AB mag ) , and the z \simeq 8 LF down to M _ { 1500 } = -17.00 .
Fitting a Schechter function , we find M _ { 1500 } ^ { * } = -19.90 ^ { +0.23 } _ { -0.28 } , \log \phi ^ { * } = -2.96 ^ { +0.18 } _ { -0.23 } , and a faint-end slope \alpha = -1.90 ^ { +0.14 } _ { -0.15 } at z \simeq 7 , and M _ { 1500 } ^ { * } = -20.12 ^ { +0.37 } _ { -0.48 } , \log \phi ^ { * } = -3.35 ^ { +0.28 } _ { -0.47 } , \alpha = -2.02 ^ { +0.22 } _ { -0.23 } at z \simeq 8 .
These results strengthen previous suggestions that the evolution at z > 7 appears more akin to ‘ density evolution ’ than the apparent ‘ luminosity evolution ’ seen at z \simeq 5 - 7 .
We also provide the first meaningful information on the LF at z \simeq 9 , explore alternative extrapolations to higher redshifts , and consider the implications for the early evolution of UV luminosity density .
Finally , we provide catalogues ( including derived z _ { phot } , M _ { 1500 } and photometry ) for the 100 most robust z \simeq 6.5 - 11.9 galaxies in the HUDF used in this analysis .
We briefly discuss our results in the context of earlier work and the results derived from an independent analysis of the UDF12 data based on colour-colour selection ( Schenker et al .
2013 ) .