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

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 imaging in the UltraVISTA/COSMOS and UKIDSS UDS/SXDS fields. The deep near-infrared imaging available in the two independent fields, in addition to deep optical (including z'-band) data, enables the sample of z ~ 6 star-forming galaxies to be securely detected long-ward of the break (in contrast to several previous studies). We show that the expected contamination rate of our initial sample by cool galactic brown dwarfs is < 3 per cent and demonstrate that they can be effectively removed by fitting brown dwarf spectral templates to the photometry. At z ~ 6 the galaxy surface density in the UltraVISTA field exceeds that in the UDS by a factor of ~ 1.8, indicating strong cosmic variance even between degree-scale fields at z > 5. We calculate the bright end of the rest-frame Ultra-Violet (UV) luminosity function (LF) at z ~ 6. The galaxy number counts are a factor of ~1.7 lower than predicted by the recent LF determination 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 of dM* ~ 0.4 mag, and show that a double power-law or a Schechter function can equally well describe the LF at z = 6. Furthermore, the bright-end of the LF appears to steepen from z ~ 7 to z ~ 5, which could indicate the onset of mass quenching or the rise of dust obscuration, a conclusion supported by comparing the observed LFs to a range of theoretical model predictions.