First discoveries of<i>z</i> ∼ 6 quasars with the Kilo-Degree Survey and VISTA Kilo-Degree Infrared Galaxy survey

Abstract

We present the results of our first year of quasar search in the ongoing ESO public Kilo-Degree Survey (KiDS) and VISTA Kilo-Degree Infrared Galaxy (VIKING) surveys. These surveys are among the deeper wide-field surveys that can be used to uncover large numbers of z 6 quasars. This allows us to probe a more common population of z 6 quasars that is fainter than the well-studied quasars from the main Sloan Digital Sky Survey. From this first set of combined survey catalogues covering 250 deg 2 we selected point sources down to Z AB = 22 that had a very red i -Z (i -Z > 2.2) colour. After follow-up imaging and spectroscopy, we discovered four new quasars in the redshift range 5.8 < z < 6.0. The absolute magnitudes at a rest-frame wavelength of 1450 are between -26.6 < M 1450 < -24.4, confirming that we can find quasars fainter than M * , which at z = 6 has been estimated to be between M * = -25.1 and M * = -27.6. The discovery of four quasars in 250 deg 2 of survey data is consistent with predictions based on the z 6 quasar luminosity function. We discuss various ways to push the candidate selection to fainter magnitudes and we expect to find about 30 new quasars down to an absolute magnitude of M 1450 = -24. Studying this homogeneously selected faint quasar population will be important to gain insight into the onset of the co-evolution of the black holes and their stellar hosts.