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We have undertaken a pilot survey for faint quasi‐stellar objects (QSOs) in the UKIRT Infrared Deep Survey (UKIDSS) Ultra Deep Survey (UDS) Field using the KX selection technique. These observations exploit the very deep near‐infrared and optical imaging of this field from United Kingdom Infrared Telescope (UKIRT) and Subaru to select candidate QSOs based on their  VJK  colours and morphologies. We determined redshifts for 426 candidates using the AAOmega spectrograph on the Anglo‐Australian Telescope in service time. We identify 17 QSOs  ( M   B  ≲−23)  in this pilot survey at  z = 1.57–3.29. We combine our sample with an X‐ray‐selected sample of QSOs in the same field (a large fraction of which also comply with our KX selection) to constrain the surface density of QSOs with  K ≤ 20, deriving limits on the likely surface density of 85–150 deg −2 . We use the good image quality available from our near‐infrared imaging to detect a spatially extended component of the QSO light which probably represents the host galaxies. We also use our sample to investigate routes to improve the selection of KX QSOs at faint limits in the face of the significant contamination by compact, foreground galaxies. The brightest examples from our combined QSO sample will be used in conjunction with a large Very Large Telescope VIMOS spectroscopic survey of high‐redshift galaxies in this region to study the structures inhabited by gas, galaxies and growing supermassive black holes at high redshifts in the UKIDSS UDS.

A pilot survey for KX QSOs in the UKIDSS Ultra Deep Survey Field