Two bright z > 6 quasars from VST ATLAS and a new method of optical plus mid-infrared colour selection

We present the discovery of two z > 6 quasars, selected as i-band dropouts in the Very Large Telescope Survey Telescope ATLAS survey. Our first quasar has redshift, z = 6.31 ± 0.03, z-band magnitude, zAB = 19.63 ± 0.08 and rest frame 1450Å absolute magnitude, M1450 =-27.8 ± 0.2, making it the joint second most luminous quasar 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., in preparation. To select our quasars, we use a new 3D colour space, combining the ATLAS optical colours with mid-infrared data from the Wide-field Infrared Survey Explorer. We use iAB-zAB colour to exclude mainsequence 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 ~50 per cent. We discuss how our 3D colour space can be used to reject the majority of contaminants from samples of bright 5.7 < 6.3 quasars, replacing follow-up near-infrared photometry, whilst retaining high completeness.