First results from high redshift quasar searches in VIKING

Abstract

This thesis presents the discovery of the first luminous z & 6.5 quasars in the VISTA kilo-degree Infrared Galaxy Survey (VIKING). After some basic quality control, quasar selection is investigated via use of initial data supplementedwith detailedmodelling of the photometric and spatial distributions of stars of spectral type M, L and T, which are known to be the cause of significant contamination in quasar colour selection spaces. Optimised selection constraints are placed on detection significance and morphology and the performance of a traditional colour selection technique is compared to a Bayesian model comparison technique. The latter is found to offer a ∼10 per cent gain in completeness over traditional colour selection. Quasar candidates are ranked via Bayesian model comparison and a subset of the highest ranked objects are put forward for follow-up imaging. In June 2011, 44 high-z quasar candidates underwent deep optical i- and z- band imaging on the ESO NTT. Just 6 of these candidates were found to have optical colours consistent z & 6.5 quasars. Spectroscopic follow-up of these objects is ongoing, but thus far three new quasars have been discovered at redshifts of z=6.5, 6.7, 6.9. This discovery rate is consistentwith zero evolution in the rate of decline in quasar space density from z & 6.4. This differs fromthe latest results from UKIDSS. Further results expected from these and other surveys will begin to constrain the true nature of quasar space density evolution in the near future. The discovery of three z ≥ 6.5 quasars in VIKING is a significant highlight in the first year of VISTA science operations. These quasars will remain important probes of the high-z universe throughout the next decade. 1