  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'=25(AB)
within the 0.6 square degree overlap region between the UDS early data release
(EDR) and the optical coverage of the SXDS. Accounting for selection
incompleteness, we estimate the corresponding surface density of z >= 5 LBGs
with z'<=25(AB) to be 0.005+/-0.002 per square arcmin. 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
>~5x10^10 Msun 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 sigma_8 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 ~3x10^11 Msun at this epoch, despite the
large co-moving volume surveyed.
