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We present the results of a search for bright (-22.7 < M_UV < -20.5)Lyman-break galaxies at z ~ 6 within a total of 1.65 square degrees of imagingin the UltraVISTA/COSMOS and UKIDSS UDS/SXDS fields. The deep near-infraredimaging available in the two independent fields, in addition to deep optical(including z'-band) data, enables the sample of z ~ 6 star-forming galaxies tobe securely detected long-ward of the break (in contrast to several previousstudies). We show that the expected contamination rate of our initial sample bycool galactic brown dwarfs is < 3 per cent and demonstrate that they can beeffectively removed by fitting brown dwarf spectral templates to thephotometry. At z ~ 6 the galaxy surface density in the UltraVISTA field exceedsthat in the UDS by a factor of ~ 1.8, indicating strong cosmic variance evenbetween degree-scale fields at z > 5. We calculate the bright end of therest-frame Ultra-Violet (UV) luminosity function (LF) at z ~ 6. The galaxynumber counts are a factor of ~1.7 lower than predicted by the recent LFdetermination by Bouwens et al.. In comparison to other smaller area studies,we find an evolution in the characteristic magnitude between z ~ 5 and z ~ 7 ofdM* ~ 0.4 mag, and show that a double power-law or a Schechter function canequally well describe the LF at z = 6. Furthermore, the bright-end of the LFappears to steepen from z ~ 7 to z ~ 5, which could indicate the onset of massquenching or the rise of dust obscuration, a conclusion supported by comparingthe observed LFs to a range of theoretical model predictions.

The galaxy luminosity function at z ~ 6 and evidence for rapid evolution in the bright end from z ~ 7 to 5