Photometric redshifts and cluster tomography in the ESO Distant Cluster Survey ^*,**

¹ Laboratoire d'Astrophysique de Toulouse-Tarbes, CNRS, Université de Toulouse, 14 avenue Édouard Belin, 31400 Toulouse, France e-mail: roser@ast.obs-mip.fr
² Leo Goldberg Fellow, National Optical Astronomical Observatory, 950 North Cherry Avenue, Tucson, AZ 85721, USA
³ Max-Planck-Institut für Astrophysik, Karl-Schwarschild-Str. 1, Postfach 1317, 85741 Garching, Germany
⁴ INAF – Astronomical Observatory of Trieste, via G.B. Tiepolo 11, 34143 Trieste, Italy
⁵ Herzberg Institute of Astrophysics, National Research Council of Canada, Victoria, BC V9E 2E7, Canada
⁶ Department of Physics and Astronomy, Ohio University, Athens, OH 45701, USA
⁷ Observatoire de Genève, Laboratoire d'Astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), 1290 Sauverny, Switzerland
⁸ GEPI, CNRS-UMR8111, Observatoire de Paris, section de Meudon, 5 place Jules Janssen, 92195 Meudon Cedex, France
⁹ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
¹⁰ The Royal Library/Copenhagen University Library, Research Dept., Box 2149, 1016 Copenhagen K, Denmark
¹¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, Postfach 1312, 85741 Garching, Germany
¹² School of Physics and Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD, UK
¹³ Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹⁴ Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK
¹⁵ Osservatorio Astronomico, vicolo dell'Osservatorio 5, 35122 Padova, Italy
¹⁶ SUPA, Institute for Astronomy, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
¹⁷ Astronomy Department, University of Washington, Box 351580, Seattle, WA 98195, USA
¹⁸ Institut d'Astrophysique de Paris, 98bis boulevard Arago, 75014 Paris, France
¹⁹ Sterrewacht Leiden, PO Box 9513, 2300 RA, Leiden, The Netherlands
²⁰ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721

Received: 21 July 2008
Accepted: 16 October 2009

Abstract

Context. This paper reports the results obtained on the photometric redshifts measurement and accuracy, and cluster tomography in the ESO Distant Cluster Survey (EDisCS) fields.

Aims. We present the methods used to determine photometric redshifts to discriminate between member and non-member galaxies and reduce the contamination by faint stars in subsequent spectroscopic studies.

Methods. Photometric redshifts were computed using two independent codes both based on standard spectral energy distribution (SED) fitting methods ( and Rudnick's code). Simulations were used to determine the redshift regions for which a reliable determination of photometric redshifts was expected. The accuracy of the photometric redshifts was assessed by comparing our estimates with the spectroscopic redshifts of ∼1400 galaxies in the  domain. The accuracy expected for galaxies fainter than the spectroscopic control sample was estimated using a degraded version of the photometric catalog for the spectroscopic sample.

Results. The accuracy of photometric redshifts is typically , depending on the field, the filter set, and the spectral type of the galaxies. The quality of the photometric redshifts degrades by a factor of two in between the brightest () and the faintest (–24.5) galaxies in the EDisCS sample. The photometric determination of cluster redshifts in the EDisCS fields using a simple algorithm based on z_phot is in excellent agreement with the spectroscopic values, such that 0.03–0.04 in the high-z sample and in the low-z sample, i.e. the z_phot cluster redshifts are at least a factor ~ more accurate than the measurements of z_phot for individual galaxies. We also developed a method that uses both photometric redshift codes jointly to reject interlopers at magnitudes fainter than the spectroscopic limit. When applied to the spectroscopic sample, this method rejects ∼ of all spectroscopically confirmed non-members, while retaining ≳ of all confirmed members.

Conclusions. Photometric redshifts are found to be particularly useful for the identification and study of clusters of galaxies in large surveys. They enable efficient and complete pre-selection of cluster members for spectroscopy, allow accurate determinations of the cluster redshifts based on photometry alone, and provide a means of determining cluster membership, especially for bright sources.