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A complete search for redshift z>6.5 quasars in the VIKING survey

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Title: A complete search for redshift z>6.5 quasars in the VIKING survey
Authors: Barnett, R
Warren, SJ
Cross, NJG
Mortlock, DJ
Fan, X
Wang, F
Hewett, PC
Item Type: Journal Article
Abstract: We present the results of a new, deeper, and complete search for high-redshift $6.5<z<9.3$ quasars over 977deg$^2$ of the VISTA Kilo-Degree Infrared Galaxy (VIKING) survey. This exploits a new list-driven dataset providing photometry in all bands ZYJHKs, for all sources detected by VIKING in $J$. We use the Bayesian model comparison (BMC) selection method of Mortlock et al., producing a ranked list of just 21 candidates. The sources ranked 1, 2, 3 and 5 are the four known $z>6.5$ 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 Council
Science and Technology Facilities Council (STFC)
Science and Technology Facilities Council (STFC)
Funder's Grant Number: ST-N000838
Keywords: astro-ph.GA
Astronomy & Astrophysics
0201 Astronomical and Space Sciences
Notes: Accepted for MNRAS
Publication Status: Published
Online Publication Date: 2020-12-10
Appears in Collections:Physics