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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.

Abstract

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.
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Status:Peer-reviewed
Publisher Web site:http://dx.doi.org/10.1093/mnrasl/slv057
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

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