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

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.

arXiv commentAccepted for MNRAS

References in corpus (52)

• Stellar population synthesis at the resolution of 2003
• The Wide-field Infrared Survey Explorer (WISE): Mission Description and Initial On-orbit Performance
• Overview of the DESI Legacy Imaging Surveys
• A luminous quasar at a redshift of z = 7.085
• 3D-HST+CANDELS: The Evolution of the Galaxy Size-Mass Distribution since $z=3$
• The COSMOS2015 Catalog: Exploring the 1<z<6 Universe with half a million galaxies
• Evidence for Reionization at z ~ 6: Detection of a Gunn-Peterson Trough in a z=6.28 Quasar
• Preliminary Results from NEOWISE: An Enhancement to the Wide-field Infrared Survey Explorer for Solar System Science
• The UKIRT Infrared Deep Sky Survey ZYJHK Photometric System: Passbands and Synthetic Colours
• The Pan-STARRS1 Surveys
• The Hawaii Infrared Parallax Program. I. Ultracool Binaries and the L/T Transition
• The Canada-France High-z Quasar Survey: nine new quasars and the luminosity function at redshift 6
• Unusual Broad Absorption Line Quasars from the Sloan Digital Sky Survey
• The Luminosity and Mass Functions of Low-Mass Stars in the Galactic Disk: II. The Field
• Pōniuā'ena: A Luminous $z=7.5$ Quasar Hosting a 1.5 Billion Solar Mass Black Hole
• The VISTA Deep Extragalactic Observations (VIDEO) Survey
• The Final SDSS High-Redshift Quasar Sample of 52 Quasars at z>5.7
• The Sloan Digital Sky Survey Data Release 7 Spectroscopic M Dwarf Catalog I: Data
• An ALMA [CII] survey of 27 quasars at z>5.94
• Discovery of three z>6.5 quasars in the VISTA Kilo-degree Infrared Galaxy (VIKING) survey
• Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). V. Quasar Luminosity Function and Contribution to Cosmic Reionization at z = 6
• The third data release of the Kilo-Degree Survey and associated data products
• Rapidly star-forming galaxies adjacent to quasars at redshifts exceeding 6
• The VISTA Science Archive
• Very Strong Emission-Line Galaxies in the WISP Survey and Implications for High-Redshift Galaxies
• Subaru high-z exploration of low-luminosity quasars (SHELLQs). I. Discovery of 15 quasars and bright galaxies at 5.7 < z < 6.9
• Exploring Reionization-Era Quasars III: Discovery of 16 Quasars at $6.4\lesssim z\lesssim 6.9$ with DESI Legacy Imaging Surveys and UKIRT Hemisphere Survey and Quasar Luminosity Function at $z\sim 6.7$
• Discovery of the First Low-Luminosity Quasar at z > 7
• The identification of z-dropouts in Pan-STARRS1: three quasars at 6.5<z<6.7
• JHK Magnitudes for L and T Dwarfs and Infrared Photometric Systems
• Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). II. Discovery of 32 Quasars and Luminous Galaxies at 5.7 < z < 6.8
• Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). IV. Discovery of 41 Quasars and Luminous Galaxies at 5.7 < z < 6.9
• Eight new luminous z > 6 quasars selected via SED model fitting of VISTA, WISE and Dark Energy Survey Year 1 Observations
• Exploring Reionization-Era Quasars IV: Discovery of Six New $z\gtrsim 6.5$ Quasars with DES, VHS and unWISE Photometry
• Three new VHS-DES Quasars at 6.7 < z < 6.9 and Emission Line Properties at z > 6.5
• Luminous K-band Selected Quasars from UKIDSS
• The Discovery of a Gravitationally Lensed Quasar at z = 6.51
• First discoveries of z~6 quasars with the Kilo Degree Survey and VISTA Kilo-Degree Infrared Galaxy survey
• First Discoveries of z>6 Quasars with the DECam Legacy Survey and UKIRT Hemisphere Survey
• Euclid preparation: V. Predicted yield of redshift 7<z<9 quasars from the wide survey
• Complex Lyman Alpha Profiles in Redshift 6.6 Ultraluminous Lyman Alpha Emitters
• Probabilistic selection of high-redshift quasars
• Mg II Absorption at 2<z<7 with Magellan/FIRE, III. Full Statistics of Absorption Towards 100 High-Redshift QSOs
• Photometric brown-dwarf classification. II. A homogeneous sample of 1361 L and T dwarfs brighter than J = 17.5 with accurate spectral types
• Photometric brown-dwarf classification. I. A method to identify and accurately classify large samples of brown dwarfs without spectroscopy
• Observations of the Lyman series forest towards the redshift 7.1 quasar ULAS J1120+0641
• A 16 deg${}^2$ survey of emission-line galaxies at z<1.5 in HSC-SSP PDR1
• A new bright z=6.82 quasar discovered with VISTA: VHS J0411-0907
• The Near and Mid-infrared photometric properties of known redshift $z\ge 5$ Quasars
• A Quasar Discovered at redshift 6.6 from Pan-STARRS1
• Discovery of a very Lyman-$\alpha$-luminous quasar at z=6.62
• High-redshift quasar selection from the CFHQSIR survey

Cited by in corpus (2)

• Paving the Way for Euclid and JWST via Optimal Selection of High-z Quasars
• QUEST (Quasar Unsupervised Encoder and Synthesis Tool): A machine learning framework to generate quasar spectra