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.
The VLT Survey Telescope (VST) ATLAS is an optical ugriz survey aiming to cover ~4700deg^2 of the Southern sky to similar depths as the Sloan Digital Sky Survey (SDSS). From reduced images and object catalogues provided by the Cambridge Astronomical
Surveys Unit we first find that the median seeing ranges from 0.8 arcsec FWHM in i to 1.0 arcsec in u, significantly better than the 1.2-1.5 arcsec seeing for SDSS. The 5 sigma magnitude limit for stellar sources is r_AB=22.7 and in all bands these limits are at least as faint as SDSS. SDSS and ATLAS are more equivalent for galaxy photometry except in the z band where ATLAS has significantly higher throughput. We have improved the original ESO magnitude zeropoints by comparing m<16 star magnitudes with APASS in gri, also extrapolating into u and z, resulting in zeropoints accurate to ~+-0.02 mag. We finally compare star and galaxy number counts in a 250deg^2 area with SDSS and other count data and find good agreement. ATLAS data products can be retrieved from the ESO Science Archive, while support for survey science analyses is provided by the OmegaCAM Science Archive (OSA), operated by the Wide-Field Astronomy Unit in Edinburgh.
