Selection constraints on high redshift quasar searches in the VISTA
  kilo-degree infrared galaxy survey
  The European Southern Observatory's (ESO) Visible and Infrared Survey
Telescope for Astronomy (VISTA) is a 4-m class survey telescope for wide-field
near-infrared imaging. VISTA is currently running a suite of six public
surveys, which will shortly deliver their first Europe wide public data
releases to ESO. The VISTA Kilo-degree Infrared Galaxy Survey (VIKING) forms a
natural intermediate between current wide shallow, and deeper more concentrated
surveys, by targeting two patches totalling 1500 sq.deg in the northern and
southern hemispheres with measured 5-sigma limiting depths of Z ~ 22.4, Y ~
21.4, J ~ 20.9, H ~ 19.9 and Ks ~19.3 (Vega). This architecture forms an ideal
working parameter space for the discovery of a significant sample of 6.5 <= z
<= 7.5 quasars. In the first data release priority has been placed on small
areas encompassing a number of fields well sampled at many wavelengths, thereby
optimising science gains and synergy whilst ensuring a timely release of the
first products. For rare object searches e.g. high-z quasars, this policy is
not ideal since photometric selection strategies generally evolve considerably
with the acquisition of data. Without a reasonably representative data set
sampling many directions on the sky it is not clear how a rare object search
can be conducted in a highly complete and efficient manner.
  In this paper, we alleviate this problem by supplementing initial data with a
realistic model of the spatial, luminosity and colour distributions of sources
known to heavily contaminate photometric quasar selection spaces, namely dwarf
stars of spectral type M, L and T. We use this model along with a subset of
available data to investigate contamination of quasar selection space by cool
stars and galaxies and lay down a set of benchmark selection constraints that
limit contamination to reasonable levels whilst maintaining high
completeness...
