Journal Article

Selection constraints on high-redshift quasar searches in the VISTA Kilo-degree Infrared Galaxy survey

J. R. Findlay, W. J. Sutherland, B. P. Venemans, C. Reylé, A. C. Robin, D. G. Bonfield, V. A. Bruce and M. J. Jarvis

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 419, issue 4, pages 3354-3367
Published in print February 2012 | ISSN: 0035-8711
Published online January 2012 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.19975.x
Selection constraints on high-redshift quasar searches in the VISTA Kilo-degree Infrared Galaxy survey

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The European Southern Observatory’s (ESO) Visible and Infrared Survey Telescope for Astronomy (VISTA) is a 4-m class survey telescope for wide-field near-infrared imaging. VISTA is currently running a suite of six public surveys, which will shortly deliver their first Europe wide public data releases to ESO. The VISTA Kilo-degree Infrared Galaxy survey (VIKING) forms a natural intermediate between current wide shallow and deeper more concentrated surveys, by targeting two patches totalling 1500  deg2 in the Northern and Southern hemispheres with measured 5σ limiting depths of Z≃ 22.4,  Y≃ 21.4,  J≃ 20.9,  H≃ 19.9 and Ks≃ 19.3 (Vega). This architecture forms an ideal working parameter space for the discovery of a significant sample of 6.5 ≤ z ≤ 7.5 quasars. In the first data release, priority has been placed on small areas encompassing a number of fields well sampled at many wavelengths, thereby optimizing science gains and synergy whilst ensuring a timely release of the first products. For rare object searches, e.g. high-z quasars, this policy is not ideal since photometric selection strategies generally evolve considerably with the acquisition of data. Without a reasonably representative data set sampling many directions on the sky, it is not clear how a rare object search can be conducted in a highly complete and efficient manner.

In this paper, we alleviate this problem by supplementing initial data with a realistic model of the spatial, luminosity and colour distributions of sources known to heavily contaminate photometric quasar selection spaces, namely dwarf stars of spectral types M, L and T. We use this model along with a subset of available data to investigate contamination of quasar selection space by cool stars and galaxies and lay down a set of benchmark selection constraints that limit contamination to reasonable levels whilst maintaining high completeness as a function of both magnitude and redshift. We review recent follow-up imaging of the first VIKING high-z quasar candidates and find that the results lend considerable support for the choice of selection constraints. The methods outlined here are also applicable to rare object searches in a number of other ongoing and forthcoming projects.

Keywords: techniques: photometric; surveys; galaxies: high-redshift; quasars: general

Journal Article.  11325 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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