Photometric redshifts and cluster tomography in the eso distant cluster survey

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 (Hyperz 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 ≤ z ≤ 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 σ(δz/(1 + z)) ̃ 0.05 ± 0.01, 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 σ(δz/(1 + z)) between the brightest (I <22)) and the faintest ((I ̃ 24-24.5) 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 dz σz ̃0.03-0.04 in the high-z sample and σ ̃ 0.05 in the low-z sample, i.e. the z _phot cluster redshifts are at least a factor ̃(1+z) 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 ̃50-90% of all spectroscopically confirmed non-members, while retaining > 90% 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.