An Exploratory Search for z~6 Quasars in the UKIDSS Early Data Release
Eilat Glikman (1), Alexander Eigenbrod (2), S. G. Djorgovski (1,2),, Georges Meylan (2), David Thompson (3), Ashish Mahabal (1), Frederic Courbin, (2) ((1) Caltech, (2) EPFL, (3) LBTO, U. of Arizona)

TL;DR
This study conducted an exploratory search for high-redshift quasars (z~6-8) using UKIDSS data, but found no quasars, highlighting the importance of optical data depth for future searches.
Contribution
First exploratory search for z~6-8 quasars in UKIDSS EDR, refining selection criteria and identifying key limitations for future high-redshift quasar surveys.
Findings
No quasars detected in the survey area.
Most candidates were low-mass stars or brown dwarfs.
Optical data depth is the main limiting factor.
Abstract
We conducted an exploratory search for quasars at z~ 6 - 8, using the Early Data Release from United Kingdom Infrared Deep Sky survey (UKIDSS) cross-matched to panoramic optical imagery. High redshift quasar candidates are chosen using multi-color selection in i,z,Y,J,H and K bands. After removal of apparent instrumental artifacts, our candidate list consisted of 34 objects. We further refined this list with deeper imaging in the optical for ten of our candidates. Twenty-five candidates were followed up spectroscopically in the near-infrared and in the optical. We confirmed twenty-five of our spectra as very low-mass main-sequence stars or brown dwarfs, which were indeed expected as the main contaminants of this exploratory search. The lack of quasar detection is not surprising: the estimated probability of finding a single z>6 quasar down to the limit of UKIDSS in the 27.3 square…
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