The colour selection of distant galaxies in the UKIDSS Ultra Deep Survey Early Data Release

We investigate colour selection techniques for high-redshift galaxies in the UKIRT Infrared Deep Sky Survey (UKIDSS) Ultra Deep Survey Early Data Release (UDS EDR). Combined with very deep Subaru optical photometry, the depth (KAB= 22.5) and area (0.62 deg2) of the UDS EDR allow us to investigate optical/near-infrared selection using a large sample of over 30 000 objects. By using the B−z′, z′−K colour–colour diagram (the BzK technique) we identify over 7500 candidate galaxies at z > 1.4, which can be further separated into passive and star-forming systems (pBzKs and sBzKs respectively). Our unique sample allows us to identify a new feature not previously seen in BzK diagrams, consistent with the passively evolving track of early-type galaxies at z < 1.4. We also compare the BzK technique with the R−K colour selection of extremely red objects (EROs) and the J−K selection of distant red galaxies (DRGs), and quantify the overlap between these populations. We find that the majority of DRGs at these relatively bright magnitudes are also EROs. Since previous studies have found that DRGs at these magnitudes have redshifts of z∼ 1, we determine that these DRGs/EROs have spectral energy distributions consistent with being dusty star-forming galaxies or active galactic nuclei at z < 2. Finally, we observe a flattening in the number counts of pBzK galaxies, similar to other studies, which may indicate that we are sampling the luminosity function of passive z > 1 galaxies over a narrow redshift range.