Exploring the formation epoch of massive galaxies

We present; new results on the cosmological evolution of the near-infrared galaxy luminosity function, derived from the analysis of a new sample of galaxies selected over an area. of 0.6 square degrees froth the Early Data Release of the UKIDSS Ultra Deep Survey (UDS). Our study has exploited the multi-wavelength coverage of the UDS field provided by the new UKIDSS WFCAM K- and J-band imaging, the Subaru/XMM-Newton Deep Survey and the Spitzer-SWIRE Survey. In agreement with previous studies we find that the characteristic luminosity of the near-infrared luminosity function brightens by similar or equal to 1 magnitude between z = 0 and z similar or equal to 2. while the total density decreases by a factor 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 < 1. However; the strength of the colour bimodality is found to he a decreasing function of redshift, and seems to disappear by z > 1.5. the have also exploited this unique dataset to search for bright Lyman-break galaxies (LBGs) at z >= 5. Modelling of the optical-I-near-infrared photometry constrains the candidates' redshifts to lie in the range 5.7 < z < 5.9, and provides estimates for their stellar masses. Although the stellar mass estimates are individually uncertain, a stacking analysis suggests that the typical stellar mass of the LBG candidates is > 5 x 10(10) M-circle dot which, if confirmed, places them amongst the most massive galaxies currently known at z >= 5. It is found that Lambda CDM structure formation can produce sufficient numbers of dark matter halos at z >= 5 to accommodate our estimated number density of massive LBGs for plausible values of as and the ratio of stellar to dark matter. Moreover, it is found that; recent galaxy formation models can also account for the existence of such massive galaxies at z >= 5.