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Description
We present a new determination of the ultraviolet (UV) galaxy luminosity function (LF) at redshift z=~7 and 8, and a first estimate at z=~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 Wide Field Camera 3/infrared 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=~7 to 8 is based on a final catalogue of =~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=~7 LF extending down to UV absolute magnitudes M_1500_=-16.75 (AB mag) and the z=~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=~7, and M_1500_^*^=-20.12^+0.37^_-0.48_, log{phi}^*^=-3.35^+0.28^_-0.47_ and {alpha}=-2.02^+0.22^_-0.23_ at z=~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=~5-7. We also provide the first meaningful information on the LF at z=~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 most robust z~6.5-11.9 galaxies 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.
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