The discovery of a significant sample of massive galaxies at redshifts 5 < z < 6 in the UKIDSS Ultra Deep Survey early data release

We have exploited the large area coverage of the combined UKIDSS Ultra Deep Survey (UDS) and Subaru/XMM-Newton Deep Survey (SXDS) to search for bright Lyman-break galaxies (LBGs) at z >= 5. Using the available optical + near-infrared photometry to efficiently exclude low-redshift contaminants, we identify nine z >= 5 LBG candidates brighter than z'_AB = 25 within the 0.6-deg² overlap region between the UDS early data release and the optical coverage of the SXDS. Accounting for selection incompleteness, we estimate the corresponding surface density of z >= 5 LBGs with z'_AB <= 25 to be 0.005 +/- 0.002arcmin^-2. Modelling of the optical + near-infrared photometry constrains the candidates' redshifts to lie in the range 5.1 < 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 × 10¹⁰M_solar which, if confirmed, places them amongst the most massive galaxies currently known at z >= 5. It is found that Lambda cold dark matter structure formation can produce sufficient numbers of dark matter haloes at z >= 5 to accommodate our estimated number density of massive LBGs for plausible values of σ₈ 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. Finally, no evidence is found for the existence of LBGs with stellar masses in excess of ~=3 × 10¹¹M_solar at this epoch, despite the large comoving volume surveyed.