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Photometric redshifts and cluster tomography in the ESO Distant Cluster Survey

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 ({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 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 σ(Δ z/(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(Δ z/(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 δ z ∼0.03-0.04 in the high-z sample and δ z ∼ 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.

Based on observations collected at the European Southern Observatory, Paranal and La Silla, Chile, as part of the ESO LP 166.A-0162.

Figures 16-22, Tables 9 and 10 are only available in electronic form at

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R. Pelló, G. Rudnick, G. de Lucia, L. Simard, D. I. Clowe, et al.. Photometric redshifts and cluster tomography in the ESO Distant Cluster Survey. Astronomy and Astrophysics, 2009, 508, pp.1173-1191. ⟨10.1051/0004-6361/200810644⟩. ⟨hal-03646218⟩



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