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 Title: A complete search for redshift z>6.5 quasars in the VIKING survey Authors: Barnett, RWarren, SJCross, NJGMortlock, DJFan, XWang, FHewett, PC Item Type: Journal Article Abstract: We present the results of a new, deeper, and complete search for high-redshift $6.56.5$ quasars in this field. Additional observations of the other 17 candidates, primarily DESI Legacy Survey photometry and ESO FORS2 spectroscopy, confirm that none is a quasar. This is the first complete sample from the VIKING survey, and we provide the computed selection function. We include a detailed comparison of the BMC method against two other selection methods: colour cuts and minimum-$\chi^2$ SED fitting. We find that: i) BMC produces eight times fewer false positives than colour cuts, while also reaching 0.3 mag. deeper, ii) the minimum-$\chi^2$ SED fitting method is extremely efficient but reaches 0.7 mag. less deep than the BMC method, and selects only one of the four known quasars. We show that BMC candidates, rejected because their photometric SEDs have high $\chi^2$ values, include bright examples of galaxies with very strong [OIII]$\lambda\lambda$4959,5007 emission in the $Y$ band, identified in fainter surveys by Matsuoka et al. This is a potential contaminant population in Euclid searches for faint $z>7$ quasars, not previously accounted for, and that requires better characterisation. Issue Date: 10-Dec-2020 Date of Acceptance: 4-Dec-2020 URI: http://hdl.handle.net/10044/1/85221 DOI: 10.1093/mnras/staa3808 ISSN: 0035-8711 Publisher: Royal Astronomical Society Start Page: 1663 End Page: 1676 Journal / Book Title: Monthly Notices of the Royal Astronomical Society Volume: 501 Issue: 2 Copyright Statement: © 2020 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model). This is a pre-copy-editing, author-produced version of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. The definitive publisher-authenticated version R Barnett, S J Warren, N J G Cross, D J Mortlock, X Fan, F Wang, P C Hewett, A complete search for redshift z ≳ 6.5 quasars in the VIKING survey, Monthly Notices of the Royal Astronomical Society, Volume 501, Issue 2, February 2021, Pages 1663–1676 is available online at: https://doi.org/10.1093/mnras/staa3808 Sponsor/Funder: Science and Technology Facilities CouncilScience and Technology Facilities Council (STFC)Science and Technology Facilities Council (STFC) Funder's Grant Number: ST-N000838ST/N000838/1ST/S000372/1 Keywords: astro-ph.GAastro-ph.GAastro-ph.COastro-ph.GAastro-ph.GAastro-ph.COAstronomy & Astrophysics0201 Astronomical and Space Sciences Notes: Accepted for MNRAS Publication Status: Published Online Publication Date: 2020-12-10 Appears in Collections: PhysicsAstrophysics