A Pilot Survey for KX QSOs in the UKIDSS Ultra Deep Survey Field

TL;DR

This pilot survey used KX selection with deep UKIDSS and Subaru imaging to identify faint QSOs in the UKIDSS Ultra Deep Survey Field, providing new insights into their surface density and host galaxy properties at high redshift.

Contribution

It introduces a novel KX-based selection method for faint QSOs in the UKIDSS UDS and combines optical, near-infrared, and X-ray data to constrain QSO surface density at high redshift.

Findings

Identified 17 QSOs at z=1.57-3.29 in the pilot survey.

Estimated QSO surface density of 85-150 per square degree for K<=20.

Detected extended host galaxy components in the QSO sample.

Abstract

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<= -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 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.