The discovery of the first luminous z ∼ 6 quasar in the UKIDSS Large Area Survey

Abstract

Abstract We present the initial results from our search for high-redshift, z≳ 6, quasars using near-infrared (near-IR) data from the UKIRT Infrared Deep Sky Survey Large Area Survey. Our analysis of 106 deg2 of sky from Data Release 1 has resulted in the discovery of ULAS J020332.38+001229.2, a luminous (JAB= 20.0, JVega= 19.1, M1450=−26.2) quasar at z= 5.86. Following candidate selection from the combined IR and optical catalogue data and stacking of multiple epoch Sloan Digital Sky Survey (SDSS) data, we have obtained optical spectroscopy for the only two z≳ 6 quasar candidates. The Very Large Telescope FORS2 spectrum of ULAS J020332.38+001229.2 shows broad Lyα+ N v 1240 emission at ∼8350 Å and an abrupt continuum break due to absorption by the Lyα forest. The quasar is not present in the SDSS DR5 catalogue and the continuum spectral index of α=−1.4 (Fν∝να) is redder than a composite of SDSS quasars at similar redshifts (α=−0.5). The discovery of one z∼ 6 quasar in ∼100 deg2 in a complete sample within our selection criteria down to a median depth of YAB= 20.4 (7σ) is consistent with existing SDSS results. We describe our survey methodology, including the use of optical data from the SDSS and the highly effective procedures developed to isolate the very small surface density of high-probability quasar candidates.