Two more, bright, z > 6 quasars from VST ATLAS and WISE

Recently, Carnall et al. discovered two bright high-redshift quasars using the combination of the Very Large Telescope (VLT) Survey Telescope (VST) ATLAS and Wide-field Infrared Survey Explorer (WISE) surveys. The technique involved using the 3D colour plane i–z : z–W1 : W1–W2 with the WISE W1 (3.4 micron) and W2 (4.5 micron) bands taking the place of the usual NIR J band to help decrease stellar dwarf contamination. Here, we report on our continued search for 5.7 < z < 6.4 quasars over an ≈2 × larger area of ≈3577 deg² of the Southern Hemisphere. We have found two further z > 6 quasars, VST-ATLAS J158.6938−14.4211 at z = 6.07 and J332.8017−32.1036 at z = 6.32 with magnitudes of z_AB = 19.4 and 19.7 mag, respectively. J158.6938−14.4211 was confirmed by Keck LRIS observations and J332.8017−32.1036 was confirmed by ESO NTT EFOSC-2 observations. Here, we present VLT X-shooter visible and NIR spectra for the four ATLAS quasars. We have further independently rediscovered two z > 5.7 quasars previously found by the VIKING/KiDS and PanSTARRS surveys. This means that in ATLAS we have now discovered a total of six quasars in our target 5.7 < z < 6.4 redshift range. Making approximate corrections for incompleteness, we find that our quasar space density agrees with the Sloan Digital Sky Survey results of Jiang et al. at M_1450 Å ≈ −27. Preliminary virial mass estimates based on the C iv and Mg ii emission lines give black hole masses in the range M_BH ≈ 1–6 × 10⁹ M_⊙ for the four ATLAS quasars.