Two bright z > 6 quasars from VST ATLAS and a new method of optical plus
  mid-infra-red colour selection
  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 \pm 0.03, z band
magnitude, z_AB = 19.63 \pm 0.08 and rest frame 1450A absolute magnitude,
M_1450 = -27.8 \pm 0.2, making it the joint second most luminous quasar known
at z > 6. The second quasar has z = 6.02 \pm 0.03, z_AB = 19.54 \pm 0.08 and
M_1450 = -27.0 \pm 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
Wide-field Infrared Survey Explorer (WISE). We use i_AB - z_AB colour to
exclude main sequence stars, galaxies and lower redshift quasars, W1 - W2 to
exclude L dwarfs and z_AB - 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 ~50%. We discuss how our 3D colour space can be
used to reject the majority of contaminants from samples of bright 5.7 < z <
6.3 quasars, replacing follow-up near-infra-red photometry, whilst retaining
high completeness.
