We have undertaken a pilot survey for faint QSOs in the UKIDSS Ultra Deep Survey Field using the KX selection technique .
These observations exploit the very deep near-infrared and optical imaging of this field from 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 AAT in service time .
We identify 17 QSOs ( M _ { B } \mathrel { \raise 1.505 pt \hbox { $ \scriptstyle < $ } \kern - 6.0 pt \lower 1.72 pt% \hbox { { $ \scriptstyle \sim$ } } } -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 \leq 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 VLT VIMOS spectroscopic survey of high redshift galaxies in this region to study the structures inhabited by gas , galaxies and growing super-massive black holes at high redshifts in the UKIDSS UDS .