The evolution of the near-infrared galaxy luminosity function and colour bimodality up to z≃2 rom the UKIDSS Ultra Deep Survey Early Data Release

We present new results on the cosmological evolution of the near-infrared (near-IR) galaxy luminosity function (LF), derived from the analysis of a new sample of similar to 22000K(AB) <= 22.5 galaxies selected over an area of 0.6 deg(2) from the Early Data Release of the UKIDSS Ultra Deep Survey (UDS). Our study has exploited the multiwavelength coverage of the UDS field provided by the new UKIDSS WFCAM K- and J- band imaging, the Subaru/XMM-Newton Deep Survey and the SpitzerWide-Area Infrared Extragalactic survey. The unique combination of large area and depth provided by this newsurvey minimizes the complicating effect of cosmic variance and has allowed us, for the first time, to trace the evolution of the brightest sources out to z similar or equal to 2 with good statistical accuracy. In agreement with previous studies, we find that the characteristic luminosity of the near-IR LF brightens by similar or equal to 1 mag between z = 0 and z similar or equal to 2, while the total density decreases by a factor of similar or equal to 2. Using the rest-frame (U - B) colour to split the sample into red and blue galaxies, we confirm the classic luminosity-dependent colour bimodality at z less than or similar to 1. However, the strength of the colour bimodality is found to be a decreasing function of redshift, and seems to disappear by z greater than or similar to 1.5. Due to the large size of our sample, we are able to investigate the differing cosmological evolution of the red and blue galaxy populations. It is found that the space density of the brightest red galaxies (MK <= - 23) stays approximately constant with redshift, and that these sources dominate the bright end of the LF at redshifts z less than or similar to 1. In contrast, the brightening of the characteristic luminosity and mild decrease in space density displayed by the blue galaxy population leads them to dominate the bright end of the LF at redshifts z greater than or similar to 1..