Photometric redshifts and cluster tomography in the ESO Distant Cluster Survey

Abstract

<i>Context. <i/>This paper reports the results obtained on the photometric redshifts measurement and accuracy, and cluster tomography in the ESO Distant Cluster Survey (EDisCS) fields.<i>Aims. <i/>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.<i>Methods. <i/>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 <i>∼<i/>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. <i>Results. <i/>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 <i>z<i/><sub>phot<sub/> is in excellent agreement with the spectroscopic values, such that 0.03–0.04 in the high-<i>z<i/> sample and in the low-<i>z<i/> sample, i.e. the <i>z<i/><sub>phot<sub/> cluster redshifts are at least a factor ~ more accurate than the measurements of <i>z<i/><sub>phot<sub/> 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 <i>∼<i/> of all spectroscopically confirmed non-members, while retaining <i>≳<i/> of all confirmed members.<i>Conclusions. <i/>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.