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 similar to 1400 galaxies in the 0.3 <= z <= 1.0 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 sigma(Delta z/(1 + z)) similar to 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 sigma(Delta z/(1 + z)) between the brightest (I less than or similar to 22) and the faintest (I similar to 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 delta z similar to 0.03-0.04 in the high-z sample and delta z similar to 0.05 in the low-z sample, i.e. the z(phot) cluster redshifts are at least a factor similar to(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 similar to 50-90% of all spectroscopically confirmed non-members, while retaining greater than or similar to 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.