DISCOVERY OF THREE z > 6.5 QUASARS IN THE VISTA KILO-DEGREE INFRARED GALAXY (VIKING) SURVEY

Studying quasars at the highest redshifts can constrain models of galaxy and black hole formation, and it also probes the intergalactic medium in the early universe. Optical surveys have to date discovered more than 60 quasars up to z similar or equal to 6.4, a limit set by the use of the z-band and CCD detectors. Only one z greater than or similar to 6.4 quasar has been discovered, namely the z = 7.08 quasar ULAS J1120+0641, using near-infrared imaging. Here we report the discovery of three new z greater than or similar to 6.4 quasars in 332 deg(2) of the Visible and Infrared Survey Telescope for Astronomy Kilo-degree Infrared Galaxy (VIKING) survey, thus extending the number 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, 0.6-1.1 mag fainter than ULAS J1120+0641. Near-infrared spectroscopy revealed the Mg II emission line in all three objects. The quasars are powered by black holes with masses of similar to(1-2) x 10(9) M-circle dot. In our probed redshift range of 6.44 < z < 7.44 we can set a lower limit on the space density of supermassive black holes of rho(M-BH > 10(9) M-circle dot) > 1.1 x 10(-9) Mpc(-3). The discovery of three quasars in our survey area is consistent with the z = 6 quasar luminosity function when extrapolated to z similar to 7. We do not find evidence for a steeper decline in the space density of quasars with increasing redshift from z = 6 to z = 7.