Photometric redshifts for the CFHTLS T0004 deep and wide fields

Free access article

Issue		A&A Volume 500, Number 3, June IV 2009


Page(s)		981 - 998
Section		Cosmology (including clusters of galaxies)
DOI		http://dx.doi.org/10.1051/0004-6361/200811413
Published online		29 April 2009

A&A 500, 981-998 (2009)
DOI: 10.1051/0004-6361/200811413

Photometric redshifts for the CFHTLS T0004 deep and wide fields

J. Coupon¹, O. Ilbert², M. Kilbinger¹, H. J. McCracken¹, Y. Mellier¹, S. Arnouts³, E. Bertin¹, P. Hudelot¹, M. Schultheis⁴, O. Le Fèvre⁵, V. Le Brun⁵, L. Guzzo⁶, S. Bardelli⁷, E. Zucca⁷, M. Bolzonella⁷, B. Garilli⁶, G. Zamorani⁷, A. Zanichelli⁸, L. Tresse⁵, and H. Aussel⁹

¹ Institut d'Astrophysique de Paris, UMR 7095 CNRS, Université Pierre et Marie Curie, 98bis boulevard Arago, 75014 Paris, France
e-mail: coupon@iap.fr
² Institute for Astronomy, 2680 Woodlawn Dr., University of Hawaii, Honolulu, Hawaii, 96822, USA
³ Canada-France-Hawaii telescope, 65-1238 Mamalahoa Highway, Kamuela, HI 9674, USA
⁴ Observatoire des Sciences de l'Univers de Besançon, UMR 6213 CNRS, 41bis avenue de l'Observatoire, 25010 Besançon, France
⁵ Laboratoire d'Astrophysique de Marseille (UMR 6110), CNRS-Université de Provence, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
⁶ INAF-Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate (LC), Italy
⁷ Osservatorio Astronomicoq di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁸ IRA-INAF, via Gobetti, 101, 40129 Bologna, Italy
⁹ Service d'Astrophysique, CEA-Saclay, 91191 Gif-sur-Yvette, France

Received 25 November 2008 / Accepted 8 April 2009

Abstract
Aims. We compute photometric redshifts in the fourth public release of the Canada-France-Hawaii Telescope Legacy Survey. This unique multi-colour catalogue comprises u^*, g', r', i', z' photometry in four deep fields of 1 deg² each and 35 deg² distributed over three wide fields.
Methods. We used a template-fitting method to compute photometric redshifts calibrated with a large catalogue of 16 983 high-quality spectroscopic redshifts from the VVDS-F02, VVDS-F22, DEEP2, and the zCOSMOS surveys. The method includes correction of systematic offsets, template adaptation, and the use of priors. We also separated stars from galaxies using both size and colour information.
Results. Comparing with galaxy spectroscopic redshifts, we find a photometric redshift dispersion, $\sigma_{\Delta z/(1+z_{\rm s})}$ , of 0.028–0.30 and an outlier rate, $\vert\Delta z\vert \ge 0.15\times (1+z_{\rm s})$ , of 3–4% in the deep field at $i'_{\rm AB}$ < 24. In the wide fields, we find a dispersion of 0.037–0.039 and an outlier rate of 3–4% at $i'_{\rm AB}$ < 22.5. Beyond $i'_{\rm AB}$ = 22.5 in the wide fields the number of outliers rises from 5% to 10% at $i'_{\rm AB}$ < 23 and $i'_{\rm AB}$ < 24, respectively. For the wide sample the systematic redshift bias stays below 1% to $i'_{\rm AB}$ < 22.5, whereas we find no significant bias in the deep fields. We investigated the effect of tile-to-tile photometric variations and demonstrated that the accuracy of our photometric redshifts is reduced by at most 21%. Application of our star-galaxy classifier reduced the contamination by stars in our catalogues from 60% to 8% at $i'_{\rm AB}$ < 22.5 in our field with the highest stellar density while keeping a complete galaxy sample. Our CFHTLS T0004 photometric redshifts are distributed to the community. Our release includes 592891 ( $i'_{\rm AB}$ < 22.5) and 244701 ( $i'_{\rm AB}$ < 24) reliable galaxy photometric redshifts in the wide and deep fields, respectively.

Key words: galaxies: distances and redshifts -- cosmology: observations -- cosmology: large-scale structure of Universe

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