The discovery of a significant sample of massive galaxies at redshifts 5 < z < 6 in the UKIDSS Ultra Deep Survey Early Data Release

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Abstract
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 (18.5+/-3.5 per square degree). 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. The corresponding number density of massive LBGs at z >= 5 is found to be a factor of ~3 lower than the predicted density of suitable dark matter halos (i.e. M >~1-2x10^12 Msun) at these redshifts, and is therefore fully consistent with Lambda_CDM structure formation models. 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.

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