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Studying quasars at the highest redshifts can constrain models of galaxy andblack hole formation, and it also probes the intergalactic medium in the earlyuniverse. Optical surveys have to date discovered more than 60 quasars up toz~6.4, a limit set by the use of the z-band and CCD detectors. Only one z>6.4quasar has been discovered, namely the z=7.08 quasar ULAS J1120+0641, usingnear-infrared imaging. Here we report the discovery of three new z>6.4 quasarsin 332 square degrees of the Visible and Infrared Survey Telescope forAstronomy Kilo-degree Infrared Galaxy (VIKING) survey, thus extending thenumber from 1 to 4. The newly discovered quasars have redshifts of z=6.60,6.75, and 6.89. The absolute magnitudes are between -26.0 and -25.5, 0.6-1.1mag fainter than ULAS J1120+0641. Near-infrared spectroscopy revealed the MgIIemission line in all three objects. The quasars are powered by black holes withmasses of ~(1-2)x10^9 M_sun. In our probed redshift range of 6.44<z<7.44 we canset a lower limit on the space density of supermassive black holes of\rho(M_BH>10^9 M_sun) > 1.1x10^(-9) Mpc^(-3). The discovery of three quasars inour survey area is consistent with the z=6 quasar luminosity function whenextrapolated to z~7. We do not find evidence for a steeper decline in the spacedensity of quasars with increasing redshift from z=6 to z=7.

Discovery of three z>6.5 quasars in the VISTA Kilo-degree Infrared Galaxy (VIKING) survey