A new multifield determination of the galaxy luminosity function at z=7-9 incorporating the 2012 Hubble Ultra-Deep Field imaging

We present a new determination of the ultraviolet (UV) galaxy luminosity function (LF) at redshift z similar or equal to 7 and 8, and a first estimate at z similar or equal to 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 similar or equal to 7 to 8 is based on a final catalogue of similar or equal to 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 similar or equal to 7 LF extending down to UV absolute magnitudes M-1500 = -16.75 (AB mag) and the z similar or equal to 8 LF down to M-1500 = -17.00. Fitting a Schechter function, we find M-1500(*) = -19.90(-0.28)(+0.23), log phi* = -2.96(-0.23)(+0.18) and a faint-end slope alpha = -1.90(-0.15)(+0.14) at z similar or equal to 7, and M-1500* = -20.12(-0.48)(+0.37), log phi* = -3.35(-0.47)(+0.28) and alpha = -2.02(+0.23)(+0.22) at z similar or equal to 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 similar or equal to 5 - 7. We also provide the first meaningful information on the LF at z similar or equal to 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 similar to 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.