A lack of evolution in the very bright end of the galaxy luminosity function from z ≃ 8 to 10

We utilize deep near-infrared survey data from the UltraVISTA fourth data release (DR4) and the VIDEO survey, in combination with overlapping optical and Spitzer data, to search for bright star-forming galaxies at z greater than or similar to 7.5. Using a full photometric redshift fitting analysis applied to the similar to 6 deg(2) of imaging searched, we find 27 Lyman break galaxies (LBGs), including 20 new sources, with best-fitting photometric redshifts in the range 7.4 < z < 9.1. From this sample, we derive the rest-frame UV luminosity function at z = 8 and z = 9 out to extremely bright UV magnitudes (M-UV similar or equal to -23) for the first time. We find an excess in the number density of bright galaxies in comparison to the typically assumed Schechter functional form derived from fainter samples. Combined with previous studies at lower redshift, our results show that there is little evolution in the number density of very bright (M-UV similar to -23) LBG5 between z similar or equal to 5 and z similar or equal to 9. The tentative detection of an LBG with best-fitting photometric redshift of z = 10.9 +/- 1.0 in our data is consistent with the derived evolution. We show that a double power-law fit with a brightening characteristic magnitude (Delta M*/Delta z similar or equal to -0.5) and a steadily steepening bright-end slope (Delta beta/Delta z similar or equal to -0.5) provides a good description of the z > 5 data over a wide range in absolute UV magnitude (-23 < M-UV < -17). We postulate that the observed evolution can be explained by a lack of mass quenching at very high redshifts in combination with increasing dust obscuration within the first similar to 1 Gyr of galaxy evolution.