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

Carnall, A.C.; Shanks, T.; Chehade, B.; Fumagalli, M.; Rauch, M.; Irwin, M.J.; Gonzalez-Solares, E.; Findlay, J.R.; Metcalfe, N.

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.

Citation

Carnall, A., Shanks, T., Chehade, B., Fumagalli, M., Rauch, M., Irwin, M., …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. https://doi.org/10.1093/mnrasl/slv057

• Journal Article Type: Article
• Acceptance Date: Apr 16, 2015
• Online Publication Date: May 11, 2015
• Publication Date: Jul 21, 2015
• Deposit Date: May 28, 2015
• Publicly Available Date: Sep 4, 2015
• Journal: Monthly Notices of the Royal Astronomical Society: Letters
• Electronic ISSN: 1745-3933
• Publisher: Oxford University Press
• Peer Reviewed: Peer Reviewed
• Volume: 451
• Issue: 1
• Pages: L16-L20
• DOI: https://doi.org/10.1093/mnrasl/slv057
• Keywords: Quasars: general, Quasars: individual: ATLAS J025.6821-33.4627, Quasars: individual: ATLAS J029.9915-36.5658.
• Public URL: https://durham-repository.worktribe.com/output/1407342