A new measurement of the evolving near-infrared galaxy luminosity function out to z ~= 4: a continuing challenge to theoretical models of galaxy formation

We present the most accurate measurement to date of cosmological evolution of the near-infrared galaxy luminosity function, from the local Universe out to z ~= 4. The analysis is based on a large and highly complete sample of galaxies selected from the first data release of the UKIDSS Ultra Deep Survey. Exploiting a master catalogue of K- and z-band selected galaxies over an area of 0.7 deg², we analyse a sample of ~=50000 galaxies, all with reliable photometry in 16 bands from the far-ultraviolet to the mid-infrared. The unique combination of a large area and depth provided by the Ultra Deep Survey allows us to trace the evolution of the K-band luminosity function with unprecedented accuracy. In particular, via a maximum-likelihood analysis we obtain a simple parametrization for the luminosity function and its cosmological evolution, including both luminosity and density evolution, which provides an excellent description of the data from z = 0 up to z ~= 4. We find differential evolution for galaxies dependent on galaxy luminosity, revealing once again the `downsizing behaviour' of galaxy formation. Finally, we compare our results with the predictions of the latest theoretical models of galaxy formation, based on both semi-analytical prescriptions and full hydrodynamical simulations.