We use cookies to ensure that we give you the best experience on our website. By continuing to browse this repository, you give consent for essential cookies to be used. You can read more about our Privacy and Cookie Policy.

Durham Research Online
You are in:

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

Carnall, A. C. and Shanks, T. and Chehade, B. and Fumagalli, M. and Rauch, M. and Irwin, M. J. and Gonzalez-Solares, E. and Findlay, J. R. and Metcalfe, N. (2015) 'Two bright z > 6 quasars from VST ATLAS and a new method of optical plus mid-infrared colour selection.', Monthly notices of the Royal Astronomical Society : letters., 451 (1). L16-L20.


We present the discovery of two z > 6 quasars, selected as i-band dropouts in the Very Large Telescope Survey Telescope ATLAS survey. Our first quasar has redshift, z = 6.31 ± 0.03, z-band magnitude, zAB = 19.63 ± 0.08 and rest frame 1450 Å absolute magnitude, M1450 = −27.8 ± 0.2, making it the joint second most luminous quasar known at z > 6. The second quasar has z = 6.02 ± 0.03, zAB = 19.54 ± 0.08 and M1450 = −27.0 ± 0.1. We also recover a z = 5.86 quasar discovered by Venemans et al., in preparation. To select our quasars, we use a new 3D colour space, combining the ATLAS optical colours with mid-infrared data from the Wide-field Infrared Survey Explorer. We use iAB − zAB colour to exclude main-sequence stars, galaxies and lower redshift quasars, W1 − W2 to exclude L dwarfs and zAB − 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 per cent. 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-infrared photometry, whilst retaining high completeness.

Item Type:Article
Keywords:Quasars: general, Quasars: individual: ATLAS J025.6821-33.4627, Quasars: individual: ATLAS J029.9915-36.5658.
Full text:(VoR) Version of Record
Download PDF
Publisher Web site:
Publisher statement:This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society: Letters ©: 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Date accepted:16 April 2015
Date deposited:04 September 2015
Date of first online publication:11 May 2015
Date first made open access:No date available

Save or Share this output

Look up in GoogleScholar