Two bright z > 6 quasars from VST ATLAS and a new method of optical plus mid-infra-red colour selection

TL;DR

This paper reports the discovery of two luminous quasars at redshifts above 6 using a novel 3D optical and mid-infrared colour selection method that effectively identifies high-redshift quasars with minimal contamination.

Contribution

The authors introduce a new 3D colour space combining optical and mid-infrared data for efficient high-redshift quasar selection, reducing contamination and maintaining high completeness.

Findings

Discovered two quasars at z > 6 with high luminosity.

Developed a 3D colour selection method combining optical and WISE data.

Achieved ~50% completeness with minimal contamination.

Abstract

We present the discovery of two z > 6 quasars, selected as i band dropouts in the VST ATLAS survey. Our first quasar has redshift, z = 6.31 \pm 0.03, z band magnitude, z_AB = 19.63 \pm 0.08 and rest frame 1450A absolute magnitude, M_1450 = -27.8 \pm 0.2, making it the joint second most luminous quasar known at z > 6. The second quasar has z = 6.02 \pm 0.03, z_AB = 19.54 \pm 0.08 and M_1450 = -27.0 \pm 0.1. We also recover a z = 5.86 quasar discovered by Venemans et al. (2015, in prep.). To select our quasars we use a new 3D colour space, combining the ATLAS optical colours with mid-infra-red data from the Wide-field Infrared Survey Explorer (WISE). We use i_AB - z_AB colour to exclude main sequence stars, galaxies and lower redshift quasars, W1 - W2 to exclude L dwarfs and z_AB - W2 to exclude T dwarfs. A restrictive set of colour cuts returns only our three high redshift quasars and no contaminants, albeit with a sample completeness of ~50%. We discuss how our 3D colour space can be used to reject the majority of contaminants from samples of bright 5.7 < z < 6.3 quasars, replacing follow-up near-infra-red photometry, whilst retaining high completeness.