Abstract
We present the discovery of two z > 6 quasars, selected as i band dropouts in
the VST ATLAS survey. Our first quasar has redshift, z = 6.31 0.03, z band
magnitude, zAB = 19.63 0.08 and rest frame 1450A absolute magnitude, M1450
= -27.8 0.2, making it one of the two most luminous quasars known at z > 6.
The second quasar has z = 6.02 0.03, zAB = 19.54 0.08 and M1450 = -27.0
0.1. We also recover a z = 5.86 quasar discovered by Venemans et al. (2015,
in prep.). To select our quasars we use a new 3D colour space, combining the
ATLAS optical colours with mid-infra-red data from the WISE Space Telescope. We
use iAB - zAB colour to exclude main sequence stars, galaxies and lower
redshift quasars, W1 - W2 to exclude L dwarfs and zAB - W2 to exclude T dwarfs.
A restrictive set of colour cuts returns only our three high redshift quasars
and no contaminants, albeit with a sample completeness of \sim50%. We discuss
how less restrictive cuts in our 3D colour space can be used to reject the
majority of contaminants from samples of bright 5.7 < z < 6.4 quasars,
replacing follow-up near-infra-red photometry, whilst retaining high
completeness.
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